I think a good approach to thinking about this is to ask yourself whether you actually feel the need to do whatever action is being considered. For example, when someone says "I need to do my homework" they feel an inherent need, desire, or obligation to complete the assignment. But if you were to say instead that "My homework needs to be done," this implies that while you may realize that it is your responsibility to do your homework, you yourself do not necessarily feel the need to complete it. The windows example would fall under the "it needs to be done" category, because you may not actually be too keen on cleaning the windows, even though they are dirty. Now if you were to say "I need to clean the windows", this would suggest that you actually feel the need to clean the windows, not simply that they need cleaning. Sorry if my explanation is a little convoluted.

I think the first three sound better than the fourth, but really between those three there is no real difference as to which sounds more natural. They all sound just fine, equally formal. I suppose maybe #3 would be the most natural if I had to select one, but not by much mind you.

For immediate family you could say "famille proche" or "famille nucleaire". Acknowledge in French can usually be translated as "reconnaitre".

You're right in that typically adjectives are placed before the nouns that they modify, but oftentimes people place them afterwards when referring to organizations, institutions, companies, etc. It's the same thing when you see company names ending in Ltd. or "Limited." As for why this is custom, I really have no idea.

People commonly say that that thing "tops their list of priorities." This means that whatever it is that they are referring to takes priority over everything else.

An old wives' tale is basically an urban legend. It's a proverb passed down through generations, based upon heresay. Oftentimes they're intended for children, discouraging them from bad behavior and the like.

I suppose a fallen god would be a god that has lost his or her divinity or immortality. Kind of how a fallen angel is one who is no longer an angel, a fallen god has lost whatever it was that made him or her a god.

Yeah that sounds fine to me, though usually you'd see a "one" or "some" in there as in: "The town saw the birth of one of the most significant groups in the history of rock" or "The town saw the birth of some of the most significant groups in the history of rock" The way you have it written implies that the town gave rise to the majority of the most significant groups in the history of rock, which is unlikely.