1) I will look for you.
3) "Used to" or "was ...ing" can both be imperfect, eg "I was walking to school when I tripped over":
Another example: "I used to eat snails, but now I prefer slugs".
I was walking ---> imperfect
I tripped over ---> perfect
Usually if you use both in the same sentence it's to emphasise the sort of contrast shown above. Also if you're describing something, like a scene, or the weather, you'd usually use the imperfect.
Obviously "I was walking" can also be done using the gerund ("ing" words - in Spanish they end in "ndo" after the vowel, eg "andando") but some languages such as German don't have this conjugation so in my experience imperfect is a much better tense to you if you want to be multi-lingual because people tend to get less confused.
It might be worth writing sentences like the ones above and then translating them into Spanish. That's what my teacher at school made me do with French, and weirdly it helped. Even a dictionary might be useful because they often have a section in the middle full of useful verb tables and example sentences.
Good luck! :)