만약 means ‘if’ and is used in front of a verb or a phrase. It is used together with (으)면 to make the meaning more clear. If a sentence is really short like the ones below, you don’t necessarily have to use 만약, only if it is really long because that way the person you are talking to will know from the start that there will be an ‘if’ in your sentence.
오늘 가면… / If I go today…
만약 오늘 가면… / If I go today…
오늘 = today
가다 = to go
You can make a sentence in the past tense as usual and then add (으)면 and also 만약 if you want to. For the future tense it’s a bit different in the sense that it’s not ~(을)ㄹ 거예요 anymore, but ~(을)ㄹ 거면.
그 영화를 봤으면… / If you’ve seen the movie…
그 영화를 볼 거면… / If you’re going to watch that movie…
영화 = movie
보다 = to see, watch