It depends. If the question is "Why are buildings tall?" it would be incorrect to say because the builders made them that way. But if you said "Because, buildings need to hold many people, at least most of the time, so most of them are tall," it would be correct.
Yes, starting a sentence with "then" is grammatically correct, especially when showing a sequence of events or actions. However, it is usually used sparingly to avoid overuse and maintain variety in sentence structure.
No
No, a sentence can begin with any kind of word. The first letter of a sentence is always capitalized to mark the start of the individual sentence, not because the word is a proper noun.
No, it is not proper english. I would try to avoid putting Because at the beginning of a sentence as well. Try This is true because... or The reason for this is...
No, except at the beginning of a sentence because it is n ot a proper n ou n.
The sentence should begin with the definite article 'the.
Unless they are proper nouns or begin a sentence, no.
"Lionel" is a proper noun because it is the name of a specific person or thing.
Yes, because "after" is a word that can begin a dependent clause. For example: After I passed all of my exams, I was able to graduate. After looking at the shirts, she selected one with blue stripes. (A dependent clause that begins with "after" needs an independent clause to complete it; the independent clause tells what happened next.)
Starting a sentence with "for" can be grammatically correct, especially in more formal or literary contexts. However, it is not commonly used in everyday spoken or written English. It can give a sentence a dramatic or poetic effect.
It may be. There is no word in English that cannot begin or end a sentence. The idea that certain word are unfit to end a sentence comes from Latin grammar, not English.
Therefore, because it is not improper, one may begin a sentence with "therefore."