N4
もし
If (For emphasis)
もしまだ頭が痛いなら、医者に行った方がいいよ。
If your head still hurts, you should see a doctor.
Explanation
「もし」 means 'if'. It goes at the start of a sentence to signal that what follows is only a supposition. It does not make the condition by itself; the conditional clause must end in 「〜たら」, 「〜ば」, 「〜なら」 or 「〜ても」: 「もし雨が降ったら、中止します」 (If it rains, we will cancel), 「もし一億円あったら、何に使いますか」 (If you had 100 million yen, what would you use it for?). It suits unlikely suppositions or 'in case something happens'.
Mixed up with
When 「もし」 opens a sentence, look for 「〜たら」, 「〜ば」, 「〜なら」 or 「〜ても」 at the end.
「もし」 is a neutral supposition, good or bad. 「万一」 is a very unlikely and usually bad event.