THere is not one singe answer to the two questions. You are talking about Phrasal Verbs, where the verb has an added particle, or preposition, and the two words together mean something more than each word separately.
Help out - means to help someone who is in a difficult situation.
Take off is not the same as take + off. TO take off is to remove an article of clothing or it can also mean the departure of a plane.
THis is a very long and complicated topic. You need to study all of these expressions in context as they occur.