Once for "that" and once for "the".

There is a "tips and notes" section for nearly every skill on the Duolingo website https://www.duolingo.com/ .

People who just use a mobile app are missing out on a lot of information that has already been prepared and explained.

A German course maintainer has even said that they think the app should only be used for revision/practising, not for the first learning, because it's really hard to learn German just with the lessons without looking at the notes and explanations.

Even if you don't use the website to learn, I recommend that you at least visit the skill in a browser once. (If you use your mobile device's browser, you might have to set it to Desktop mode for the tips and notes to appear.)

To access the tips and notes, click on a unit to select it, and then on the lightbulb on the little window that pops up:

Also, if you're using a mobile app, see whether you have a little lightbulb icon -- some randomly-selected mobile users do have access to tips and notes through the app as part of an A/B test, from what I have heard.

See the notes to skills Basics 1 and Basics 2. "van" is not used when you are describing what something is like (and, in particular, what kind of thing something is).

Because ez is "this", but the English sentence uses "that".

I think you mean the word "vagy", it is used when talking to someone in order to describe a certain condition, like when saying "Jól vagy?" meaning "Are you ok?" or "Hol vagy?" meaning "Where are you?" or "Késön vagy" meaning "You are late". Thus the sentence "Az vagy asztal?" is grammatically wrong, also if you say it you would be asking a table, you certainly don't want to do that :)