These are good Qs, but I recommend that you join reddit r/Korean, put them on that site.
you'll get answers from native speakers and advanced people, and you'll get answers fast. Evita is amazing for making these decks and content, of course, but I don't want her to have to be like a tutor for all of us learners. The subreddit I'm recommending is full of enthusiastic learners who will encourage you and answer even basic Qs.
I'll answer your Qs here (but bear in mind I'm just intermediate)
이거 = contracted form of 이것 (short contracted forms will esp appear a lot in speech and informal text msgs)
좋다 vs 좋아 하다
I'll explain it simply, but I'll warn you against transliterating directly into English. Remember that Korean does not state subjects and objects explictily when the subjects and objects are understood by the speaker and listener. So it's reasonable to translate 이 파자가 좋다 as I like this pizza! even though the word "I" doesn't appear in the Korean sentence. Literally, the sentence says This pizza is good! But because Korean sentences often omit nouns when they're understood contextually, a perfectly competent translator (or a dubber on a TV show etc) would be perfectly reasonable translating the sentence as I like this pizza.
So here's the simple explanation:
이 파자가 좋다! = I like this pizza! (lit. This pizza is good! )
이 파자를 좋아 해
! = I like this pizza!
The shorter one is probably more common in spoken Korean. The word "I" appears in neither sentence; it would be contextually understood.
As an English speaker taking English thoughts and putting them in Korean, you're going to be trying to say things like "나는 이 파자를 좋아 해" (I like this pizza) but you should resist the temptation to add 나는 to your sentences. When speaking in Korean, we don't say "I" in our sentences when we already know in the conversation who you're talking about. You might say it at the beginning of a bunch of sentences but aren't grammatically required to say "I" in all of your sentences as you would in English, where grammar requires every sentence to have a stated subject noun.
Korean uses lots of "understood subjects"; English does so only in one situation... the imperative mood: "Close the door" (the "you" is contextually understood).
(A more advanced speaker might disagree w/ some part of my comments here; that's why reddit is better... ppl will try to help you and smarter people will refine those explanations.)