The PRESENT TENSE uses the verb's base form (write, work), or, for third-person singular subjects, the base form plus an -s ending (he writes, she works).

The PRESENT TENSE indicates that an action is present, now, relative to the speaker or writer. Generally, it is used to describe actions that are factual or habitual -- things that occur in the present but that are not necessarily happening right now: "It rains a lot in Portland" is a kind of timeless statement. Compare that to the present progressive -- "It is raining in Portland" -- which means that something is, in fact, going on right now. "I use my bike to get around town." is in the present, but I'm not actually on my bike right now. An instantaneous sense of the present can be conveyed with either the simple present or the progressive: "Watch him now: he holds [is holding] down the control key at the same time that he presses [is pressing] the letter d."

The present tense is used to describe events that are scheduled (by nature or by people): "High tide is at 3:15 p.m. The Super Bowl starts at 6:15 p.m."

The present tense can be used to suggest the past with what is sometimes called the fictional (or historic) present: "We were watching the back door when, all of a sudden, in walks Dierdre." With verbs of communicating, the present tense can also suggest a past action: "Dierdre tells me that she took her brother to the dentist." Most oddly, the present tense can convey a sense of the future, especially with verbs such as arrive, come, and leave that suggest a kind of plan or schedule: "The train from Boston arrives this afternoon at two o'clock."

Authority for this section: A University Grammar of English by Randolph Quirk and Sidney Greenbaum. Longman Group: Essex, England. 1993. Used with permission.

Present tense habitual activities are frequently signaled by time expressions such as the following:
all the time
every class
every day
every holiday
every hour
every month
every semester
every week
every year
most of the time