Handling verbs in past, present and future is much easier in sign language than in most spoken languages.
Most of the time, context will dictate the tense of a verb. If the person is talking about his vacation to the mountains last week then all his verbs are in the past tense. When you tell a story about last Christmas you don’t need to look for a way to add -ed to all your verbs. Context already tells your friends you are speaking in past tense.
Occasionally you will need to specify that you are talking about something in the future or past. This is easily done by adding the sign for FUTURE/WILL or PAST/BEFORE to your sentence. You can do the same thing for the present by adding the sign NOW. The addition of the PAST, PRESENT and FUTURE signs to a sentence can be added at the very front or end of a sentence in ASL. State what you have to say and then add PAST, NOW or FUTURE signs to the sentence and you have added tense to all your verbs.