Present tense is used, in principle, to refer to circumstances that exist at the present time (or over a period that includes the present time) and general truths. However the same forms are quite often also used to refer to future circumstances, as in "He's coming tomorrow" (hence this tense is sometimes referred to as present-future or nonpast). For certain grammatical contexts where the present tense is the standard way to refer to the future, e.g., conditional sentences and dependent clauses. It is also possible for the present tense to be used when referring to no particular real time (as when telling a story), or when recounting past events (the historical present, particularly common in headline language). The present perfect intrinsically refers to past events, although it can be considered to denote primarily the resulting present situation rather than the events themselves.
This is a companion discussion topic for the original entry at http://grammarerror.com/tags/verb-tense