Just a note, those aren't sentences in English. As for how to say those things in Japanese, I would think it would depend on what context you're thinking of. What exactly is it you wanted to use these phrases for?
Paypal me ¥1000 to paytyt@yahoo (.com). I will do all of those questions. Then we can set up some online Skype courses. I have lived in Japan 17 years. If your level gets to high for me to teach you... I can let my wife take over the lesson.
As you may have noticed, Japanese is very dependent on context.
In English we are careful to use different verb forms for singular and plural items (is/are , was/were, do/does, etc) but in Japanese no one gives a d****. Native speakers don't even think about it. So, "This is a pen" and "These are pens" are identical in Japanese, (except when some pedant tries to translate the plurality by using un-natural Japanese).
There is often no stated subject in sentences because verbs are chosen according to who executes them.
"I gave him a pen" could have a number of translations depending on the relationship between the giver and the receiver, and even the writer.
However the verb used for "to be" varies according to something else. You can use Google translate on these and spot the rule for yourself.
There is a box.
There is a bird.
There is a flower.
If you can explain how you intend to use the answers to your questions, maybe we can give you foolproof answers, but without context, we can't do any better than the pig-in-a-poke of machine translation. That, BTW is why MT is so bad between English and Japanese; we understand context.
When you mean ”how many years have you studied Japanese ”, we’d usually say ”nan nen benkyo shitemasuka”. We can understand it if you say “nan nen kan”, though.
When you mean ”which year were you born?”, I think we’d say ”nan nen umare?”. Or ”nan nen ni umaremashitaka”
Similar to "Nan nen?", "Nan nichi?" can mean both "How many days?" and "Which day?". See the following example.
Raishū oyasumi o itadakitai n desu ga. B: 何日？
In this case, B's reply "Nan nichi?" is ambiguous. It can mean both "How many days do you need?" and "Which day do you want to take off?", thus, it's hard to say that "Nan nichi?" is the best chice for "How many days?" here. Instead, "Itsu?" works better as "Which day?" than "Nan nichi?", and similarly, "Dore kurai?" can be better as "How many days?" than "Nan nichi?". Furthermore, there are many variations for "Nan nichi?" in the level of politeness. The most appropriate expression differs depending on the relation between the speaker and the addressee, for instance, there are cases where the polite form "Nan nichi desu ka?" should be used. This is exactly why the context is needed for the correct answer, as Julie-san pointed out.