Learn Danish: Jeg ser en mand.

In Danish, every noun has a gender: common or neuter. mand is a common gender noun, so it uses the article en (meaning "a"). If it were a neuter noun, you would use et instead.

Does the verb ser change if the subject is "he" or "we" instead of Jeg?

No, Danish verbs are very easy in the present tense! They almost always end in -r and never change based on who is doing the action. It is jeg ser (I see), han ser (he sees), and vi ser (we see).

Is the 'd' in mand pronounced?

No, the 'd' is silent. In Danish, a 'd' that comes right after an 'n' is usually not pronounced. The word mand sounds very much like the English word "man".

If I wanted to say "one man" instead of "a man", is the word still en?

Yes, but with a tiny difference in writing and speech. The number "one" is written as én (with an accent) and gets more stress when spoken. The article "a" is just en and is spoken quickly without stress.