You can use taan and kin interchangeably. But if you're talking to your close friends or family, kin totally suffice. And if you want the sentence to sound lighter or softer, you can use taan.
So if you were to run into your friend, you might say "Hey, did you eat anything yet?/Did you have your lunch yet?", that's equivalent to "เป็นไง กินข้าวมายัง" [Pen Ngai, Kin Khao Ma Young. Lit. What's up? Did you eat yet?] I should note though, that when you ask someone if they had eaten, it's the same thing as when you say "Hi, how are you doing?" Apparently, we care a lot of a food or something.
Spelling in Thai is all about pure memorizing. (Even I have troubles with it and need to look up the correct spellings sometimes) Words pronounce the same way, but spell in different ways will always different meanings.
Let's like "baat/baad" for example:
บาด - Wounded or cut
บาตร - Refers to the bowl that the monks carry in the morning (when we "tak baat")
บาท - As in the Thai currency 'Baht'
บาศ - A rope or lasso
All 4 pronounce exactly the same way ;_;! So it really is about memorizing and get used to it. If you see it often, it becomes a second nature I suppose.
And about constructing sentences, personally I feel that it's a lot easier in Thai. We don't have tenses or plural or blah blah crazy grammar rules to worry about. I don't know if I should get into it now though, this is a long reply, lol