getting 404 error when I use `'/'` instead of `'/cards'` and `'/cards:id'` doesn't work nor does `cards[req.params.id]`

I discovered another error that prevents me from moving on from this video. I had to change the app.use('/cards', cardRoutes); to app.use(cardRoutes);. Now my new problem is this:

When I have my cards.js file set up like this:

const express = require('express');
const router = express.Router();
const {data} = require('../data/flashcardData.json');
const {cards} = data;

router.get('/cards', (req, res) => {
  res.render('card', {
    prompt: cards[0].question,
    hint: cards[0].hint
  });
});

module.exports = router;

it works and I can view the cards page finally. But in the video Andrew said we can just change router.get('/cards', (req, res) => { to router.get('/', (req, res) => { and it will still work. When I change mine to just have '/' it stops working and I get a 404 error so I left it as '/cards' and then it works again. So I followed along in the next video but as soon as I tried doing what he said: '/cards:id' and prompt: cards[req.params.id].question, hint: cards[req.params.id].hint then it stopped working and I got a 404 error again when I typed this into the url: localhost:3000/cards/0.

Can anyone please help me to understand what is going on? Andrews videos are really confusing. I wish Guil was teaching this stuff because I'm really interested in Node and Express and all the server side stuff, and Guil is meticulous and doesn't skip over anything or assume we know something. He just gives it all to us, simple, concise, and meticulous with the details.