That depends upon how it's pronounced when read aloud. In this particular case, the individual letters of the abbreviation NGO will be pronounced: en-gee-oh. Therefore, use anto introduce it, because of the initial vowel sound.
If the letters form an acronym -- an abbreviation that can be pronounced as if it were a word -- that's another kettle of fish. In that case, use the indefinite article that goes with how the acronym is pronounced. For example, you would write a RAM chip, not an RAM chip, because people read the acronym RAM as they would the word ram, not R-A-M.
The preferred usage seems to be, "with compliments of ...."
You should say for your convenience it is *gramatically correct-*just means correct
be rest assured
No there shouldn't be an apostrophe in this case. It should be: The 1400s.
As in, for example, 'it was the childrens' toy', as in it belonged to everyone? If so, yes.
The correct usage is "an NGO." The choice between "a" and "an" depends on the sound that follows it, not just the letter. In this case, "NGO" starts with a vowel sound (pronounced as "en-jee-oh"), so "an" is used before it. It's based on the pronunciation, not specifically the letter "N" in "NGO."
Technically, you could use retrospect as a verb, but it is not good usage. It would be better to say "I remember."
The preferred usage seems to be, "with compliments of ...."
The sentence is gramatically correct.
yes.
Everybody has is correct.
Both are correct, but "There are a lot of" is more commonly used when referring to a plural noun. For example, "There are a lot of books on the shelf."
Yes, you can begin a sentence with the word "or" when presenting options or alternatives. For example, "Or you can choose to study abroad for a semester."
At dawn the hikers set out.
Yes it is.
No 'It's for you' would be though
Yes, that answer is gramatically correct