There are two sets of endings: past and present, which are used in conjugating verbs in different tenses. These two sets happen to only differ in 3rd person singular (3S):
NOTE These endings are enclitic. That is, contrary to a suffix, they are unstressed and don't change the stress position. In Persian, the stress position is on the last syllable but these endings, being enclitic, don't bring the stress on the last syllable. The stress thus falls on the preceding syllable. Example: nevesht+am = ne.vesh.tam (instead of, ne.vesh.tam).
In English, we can't omit the «I» in «I go to school» and say «go to school» because the person of the verb becomes unclear; «go» can refer to «I, you, we, they». But in Persian, each person has a unique ending. For example, in «raftam», the ending «-am» indicates that the person of the verb is «I». Languages having such a feature are called «pro-drop» (from «pronoun-dropping»). In Persian, subject pronouns are only used when we want to put some emphasis and we don't normally say «man raftam» (it sounds like «I do go»).