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The present tense is a grammatical tense whose principal function is to locate a situation or event in the present time. The present tense is used for actions which are happening now. In order to explain and understand present tense, it is useful to imagine time as a line on which the past tense, the present and the future tense are positioned. The term present tense is usually used in descriptions of specific languages to refer to a particular grammatical form or set of forms; these may have a variety of uses, not all of which will necessarily refer to present time. For example, in the English sentence "My train leaves tomorrow morning", the verb form leaves is said to be in the present tense, even though in this particular context it refers to an event in future time. Similarly, in the historical present, the present tense is used to narrate events that occurred in the past.