Introducing Looplines: Learn English from YouTube Videos

If you've ever tried learning English by watching YouTube, you probably know the drill: you hear a word you don't know, pause the video, open a new tab, google it, lose your place, and eventually just give up and turn on Chinese subtitles.

We've all been there. That's why we built Looplines.

Not another flashcard app. Not another grammar textbook. Just a simple tool that lets you learn while you watch. Paste a YouTube link, and every word in the subtitles becomes clickable. That's the core idea. The AI chat, spaced repetition, exercises — all of that grew from that one thing.

How it actually works

Here's what using Looplines looks like day to day:

1. Paste a YouTube URL

Go to looplines.app, paste any YouTube video link, hit enter. The video loads with interactive subtitles — every single word is clickable.

2. Click any word you don't know

Say you're watching a TED talk and the speaker says "serendipity." Click it. You'll instantly see:

• The definition and pronunciation (with audio playback)
• Chinese translation
• Example sentences showing how it's used
• Whether it shows up in exams (CET-4, CET-6, IELTS, TOEFL, etc.)

No tab-switching. No copying and pasting into a dictionary. Just click.

3. Select phrases, not just single words

Heard "kick the bucket" and confused why someone's kicking buckets? Hold Shift and click to select the whole phrase. The AI will explain it's an idiom meaning "to die" — with context from the actual video you're watching, not some generic dictionary entry.

Looplines also automatically detects phrasal verbs like "look up," "give in," and "break down," so you don't have to guess which words go together.

4. Save words and review them later

Hit the + button to save any word to your vocabulary list. Looplines uses SM-2 spaced repetition — the same algorithm behind Anki — to schedule reviews. You'll see words again right when you're about to forget them.

Your vocabulary list shows word status at a glance: New, Learning, or Learned. You can filter by what's due for review, sort by date or alphabetically, and export to CSV, JSON, or Anki format.

5. Practice with AI exercises

From any video, generate grammar drills, cloze tests, and sentence reconstruction exercises. The exercises use real sentences from the video you just watched, so you're practicing with content you actually care about — not random textbook examples.

Pick your difficulty (Easy, Medium, Hard) and how many questions you want (3, 5, or 10). The dashboard tracks your accuracy over time.

A real example

Let's say you're watching a Kurzgesagt video about black holes. The narrator says:

"The gravitational pull is so immense that not even light can escape."

You click "immense" — definition pops up: extremely large or great. You click "gravitational pull" — the AI explains it in context. You save both words.

Later, in the Practice tab, you get:

The _______ pull is so _______ that not even light can escape.

Fill in "gravitational" and "immense." Done. You just learned two words from a video you actually enjoyed watching.

That's the loop: Watch → Click → Save → Practice → Remember.

What's in the box

Here's what Looplines can do right now:

• Interactive subtitles — click any word for instant definitions and translations
• AI explanations — deep contextual breakdowns of words, phrases, and grammar points
• AI chat — ask the AI anything about the video ("what does 'pull it off' mean here?")
• Loop playback — set A/B points to replay tricky sections, adjust speed from 0.5x to 2x
• Vocabulary builder — save words, review with SM-2 spaced repetition
• AI exercises — grammar drills and cloze tests generated from the video you're watching
• Bilingual transcripts — toggle between English, Chinese, or side-by-side bilingual view
• Phrasal verb detection — automatically identifies multi-word verbs in subtitles
• Caption search — search across YouTube captions to see how native speakers actually use a word
• Keyboard shortcuts — R to replay a line, L to loop it, [ / ] to set loop points, 1-6 for speed presets
• Timestamped notes — attach notes to specific moments in any video
• Learning dashboard — track vocabulary growth, exercise accuracy, and study streaks
• Offline-first — all your data stays on your device, no account required
• Dark mode and 6 accent color themes

Good channels to start with

Not sure what to watch? These educational channels work great with Looplines:

• TED & TED-Ed — talks and animated explainers on every topic imaginable
• Kurzgesagt — science topics with beautiful animations
• Veritasium — physics and those "wait, I never thought of that" moments
• Vox — clear, well-researched explanatory journalism
• Computerphile — computer science explained for normal humans
• BBC Learning English — made specifically for English learners
• Johnny Harris — investigative journalism with great storytelling
• Wendover Productions — logistics, aviation, and how things work

The app suggests videos from these channels daily, so you always have something fresh to watch.

AI — works out of the box

Looplines has a built-in AI mode that works without any setup. Just start using the chat, explanations, and exercises — no API key needed.

If you prefer to use your own AI provider, you can plug in keys for OpenAI, OpenRouter, DeepSeek, Minimax, or any OpenAI-compatible endpoint. Configuration is stored locally on your device.

Your data stays yours

Everything — vocabulary, notes, exercise history, AI settings — is stored locally in your browser. There's no account to create, no data uploaded to our servers. The app works offline after the first load.

We only make network requests when you need them: fetching captions, looking up words in the dictionary, or calling the AI. That's it.

What's next

Looplines is still under active development. Some things we're working on:

• A native mobile app (the web version works on phones, but a dedicated app would be better)
• More exercise types
• Community word lists
• Better onboarding for new users
• Support for more language pairs beyond English ↔ Chinese

If you have ideas or run into bugs, drop us a line at hello@looplines.app.

Or just go to looplines.app, paste a video, and try it yourself.

Frequently Asked Questions

Is Looplines free?

Yes. The core features — interactive subtitles, vocabulary saving, spaced repetition review — are all free. AI features work out of the box with the built-in mode, or you can use your own API key.

Do I need to create an account?

No. Everything is stored locally in your browser. No sign-up, no email, no password.

Does it work on mobile?

Yes, the web app works on mobile browsers. The experience is better on desktop because you have more screen space for the video and study panel side by side, but mobile works fine for vocabulary review and exercises.

What languages does Looplines support?

Currently it's designed for Chinese speakers learning English — definitions include Chinese translations, and the interface supports both English and Chinese. More language pairs are planned.

Can I use it with any YouTube video?

Any YouTube video that has subtitles (auto-generated or manual). Most public videos have auto-generated English captions. If a video doesn't have captions, you can upload your own SRT or VTT file.

How is this different from just turning on YouTube subtitles?

YouTube subtitles are passive — you read them and move on. Looplines makes them interactive. Every word is clickable, you can save vocabulary, the AI explains things in context, and you can generate practice exercises. It turns passive watching into active learning.

What's the relationship between Looplines and Grab Captions?

Grab Captions downloads subtitle files. Looplines uses those same captions to create an interactive learning experience — click words, build vocabulary, practice with AI. Think of Grab Captions as the raw material and Looplines as the learning tool built on top of it.