A closer look
Tense is the form a verb takes to show when something happens. I walk to school is present; I walked to school is past. English also changes verbs for other qualities, like aspect, but tense itself is just about time.
Yesterday happened: Sarah walked through the crowd. The verb form walked shows the action is over and in the past.
Today is still going: Sarah walks through the crowd every morning. Here walks points to present, routine time. Same verb, different form, different time.
English also changes verbs for other qualities, such as aspect, but tense at its core marks time through verb form, so walked versus walk stays a tense contrast.