1) Perfect tenses are used to describe ongoing actions and their effects, whether those actions occurred in the present or in the past. Using a perfect tense means there is a consequence, or continuation, of an action. Perfect tense is also used to describe an experience that doesn't have a definite place in time. Examples: I have forgotten my notebook (and I cannot do my homework) We have eaten all the food (and now you will starve) The children have been playing together (and now they are tired) We have been visiting the museum (because we like its art) I had been cleaning my room (before I was interrupted) 2) Simple present tense is used to state information or to refer to a specific point in time. I forgot my notebook when I left work. We ate all the food before you arrived. The children played together today. We were visiting the museum this morning. Perfect present tense is used to state an experience or to refer to undefined points in time. Compare the previous sentences. 3) Past perfect tense describes an ongoing action or experience that occurred in the past. Past tense can only describe past events at a specific time. Examples: "I had been studying Japanese in college" versus "I studied Japanese in college". Both sentences have the same meaning, but the first sentence doesn't refer to a specific time while I was in college, whereas the second sentence refers to a specific time (when I was in college). "I had been cleaning my room before I was interrupted" versus "I was cleaning my room before I was interrupted". Both have the same meaning, but the first sentence describes an ongoing past action and the second sentence is only a statement about a specific time: "before I was interrupted". 4) Both refer to future events, but "to be about" will happen sooner than "to be going".

They're the same, but begin can sound more formal. I use "start" if the listener doesn't know to perform the action, "begin" otherwise. For example, I could tell my boss "Start writing" or "Start taking notes" if he doesn't realize we need to write down information. I could also say "Begin cooking" or "Begin writing" if this is a competition and I am instructing people. Begin does sound softer than Start, but in any formal situation you could certainly say "You may begin" or "You may start". I find myself using "start" more often, regardless of formality.