Some (not all) verbs are followed by "to + infinitive". The verb "decide" is one example: I decided to go to the party tomorrow. You have to learn which verbs can be followed by to + infinitive. Take a look here for more information and examples: http://learnenglish.britishcouncil.org/en/english-grammar/verbs/verbs-followed-infinitive .

That's right! "To" can also mean something like "for the purpose of". Example: "Why are you going to the store? To buy some steak for dinner.". I have a quick little correction in your question. In general, you don't use contractions with "to have to". So instead of "We've to use it". use "We have to use it." Likewise "I've to do something", should be "I have to do something".

That's correct. Example: I want to the eat the cookie. "To" can also be put in front of a verb. This is called an infinitive (to eat, to dream, etc.) Example: My desire, to eat the cookie, is very simple.

Remember, the "to" form of the verb is the infinitive, just like "hablar" is the infinitive. For the most part, we use it the same way. For example: I want TO TALK to you. In Spanish, it's: Quiero HABLAR contigo. Notice how both of those infinitive forms follow a conjugated verb.

depends on the previous word