Questions & Answers about الجار رجل مجتهد.
How would I ask "Who is the neighbor?" to get this sentence as an answer?
Since you are asking about a person's identity, you would use the question word من (man), meaning "who". In Arabic, you simply put the question word before the subject, so "Who is the neighbor?" is من الجار؟ (man al-jaar?).
Where is the word for "is" in this sentence?
Arabic doesn't use a word for "am", "is", or "are" in the present tense! To say "The neighbor is a man", you simply place the subject (الجار, the neighbor) right next to the information about him (رجل, a man). The "is" is automatically understood from the context.
Why does الجار have the prefix الـ (al-) but رجل doesn't?
The prefix الـ makes a word definite ("the"). الجار has it because we are talking about a specific person: the neighbor. رجل lacks it because it is indefinite: a man. Placing a definite noun right before an indefinite noun is the standard way to create an "A is B" sentence in Arabic.
Why does the word for "hardworking" (مجتهد) come after "man" (رجل)?
In Arabic, adjectives always come after the noun they describe. Instead of saying "a hardworking man", Arabic phrasing translates more like "a man hardworking" (رجل مجتهد). Notice that both words are indefinite (neither has الـ); adjectives must match their nouns in definiteness.