The past tense refers to actions that have already happened. For example, "she is walking" refers to a girl who is currently walking (present tense), while "she walked" refers to a girl who was walking before now (past tense).
1. No longer current; gone by; over: His youth is past. 2. Having existed or occurred in an earlier time; bygone: past events; in years past. 3. a. Earlier than the present time; ago: 40 years past.
In summary: To keep past and passed straight, remember that past always has the same form, while passed is one of the forms of the verb pass. By putting a sentence in the future tense you can see which you want.