If it is just for one day, I think one would expect to learn a few things, but leaps and bounds would not be a reasonable expectation. A lot of learning is done through trial and error, as well as memorization (which usually consists of learn, remember, forget, relearn, reinforce) therefore one could expect to see more prominent results in a week, but even then, the results would not be impressive. 

I'd say total immersion for 3 months would be enough to make one able function on a reasonable level. That's starting from zero knowledge.

In case you don't know, there's a guy who does this very thing - total immersion for 90 days. He plops himself in a country and refuses to speak English for 3 months then charts his progress. He's been very successful in many languages, though not all that he has tried. Google 'fluent in three months'

Try to think of it like this: We use doors to enter rooms (generally speaking). We must pass 'through' the door first in order to go into the room. We don't go 'in' doors. We go in/into rooms. Think of through as means of passage. It is a continuous action as opposed to in, which is static. For example: The train went through the tunnel. (train went in and came out) The train went in the tunnel and crashed. (train went in, but didn't come out - it ended up motionless) Did the burglar enter the house through a window or a door? He entered through the back door. Through in your example is the best option. You can say 'You can go in any door' and people will probably know what you mean, but grammatically speaking, it isn't correct (though lots of people use it). I hope this makes it a little more clear for you.

We use this idiom when giving information that we think we know, but might not be 100% sure about all the details. In other words, it's information we're giving from memory without the benefit of trying to actively concentrate or do research. Example: How many cases of Coke are there in the supply room? Off the top of my head, I'd say there are about six cases. Do you know how many people she invited to the party? Off the top of my head, I think she invited 50. When is the next research paper due? Just off the top of my head, I think it's due in March. Can you name all the presidents of the USA? Off the top of my head, I can only name about ten. Hope this helps.

Though you give no context to go along with these phrases, they are all collocations which are used in English. The first one, 'in a broad sense', is the most commonly used.

Era and epoch are synonyms, however era refers to a period of time of indeterminate length of usually more than one year. 'The old houses in New Orleans were built during an era when craftsmen took pride in their work, therefore the houses are of high quality.' An epoch is a particular period of history usually in which especially noteworthy events took place. 'During the Cretaceous Epoch, though dinosaurs continued to dominate the land, new forms of mammals, birds and flowering plants began to appear.' In regard to your second question, I'm not quite sure what you mean by 'structure' of the sentence. It is a complete sentence (as opposed to a fragment or run on) and it is written in the present perfect tense.

One form (saw), is in the simple past tense and the other (have seen), is in the present perfect. We use the present perfect when talking about actions that continue to be true in the present, or in other words, the time period has not finished. Also, we use the present perfect when talking about subjects when the time of the action is not important or not specified. Look at the following example: 'I saw 'Star Wars' in 1982' The action started and ended in the past. versus 'I have seen Star Wars 17 times.' It doesn't matter when you first or last saw it, you are indicating that there might be a chance you will see it again which means it is an action which began in the past and continues in the present. 'I saw John three times last week.' That time period has ended and therefore in the past. 'I have seen John three times this week.' The week is not over, therefore this action continues in the present. 'I saw that exact woman here last week on Friday.' Ended in the past on a specified day. 'I've seen that woman before.' No specifics as to when in the past but the action continues in the present. Hope this helps.

Actually, the correct spelling is 'busby'. A quick look on Google revealed this: "Possibly the name's original sense of a "busby wig" came from association with Dr Richard Busby, headmaster of Westminster School in the late seventeenth century; the later phrase buzz wig may have been derived from busby. An alternative explanation is that the British hussar cap of the early 19th century was named after the hatter who supplied the officer's version—W. Busby of the Strand London." Source: http://en.wikipedia.org/wiki/Busby

I think you have the concept a little confused. When rating something, it doesn't have to involve the sex or gender of a person. You can rate other things as well. Moreover, when rating something, we usually do it on a scale from 1-10 with 10 being the highest (good). and 1 being the lowest (bad). Here is an example: Me: I just saw the new Star Trek movie last week. You: Was it any good? Me: Yeah, I liked it. You: What would you rate it? Me: About an 8. So, in your example above, if you LIKE the way the other person looks, you'd give them a ten. Here is another example: Me: The new guy in accounting is very cute! Her: Really? Me: Yep. He's easily a 9, maybe even a 10