The present perfect is formed with the verb have and the past participle. It is another way of expressing the past.

Usually the past participle form is exactly the same as the simple past form of a verb, however many verbs have irregular past participles.

be   been go   gone come come run run swim swum fly flown see seen speak   spoken do   done give given hide hidden become   become take taken get   gotten / got break   broken

shake   shaken wake waken awake   awaken rise   risen lay lain fall   fallen begin   begun know   known forget forgotten write   written choose chosen drink   drunk eat eaten bite   bitten show shown

When have is used in the present perfect, it can be contracted with the preceding pronoun.

The present perfect is negated with not. When not is present, have can be contracted with not but it can't be contracted with the preceding pronoun.

To ask a question, have is brought to the front of the sentence.

There is some degree of overlap between the simple past and the present perfect however there are important differences. When a specific past time is specified, the simple past should be used.

When the time is unspecified, either the present perfect or the simple past can be used.

The simple past expresses more finality than the present perfect.

In the sentences above, the first sentence leaves open the possibility that John continues to teach mathematics. The second implies that John no longer teaches mathematics. Maybe he retired, changed careers, or passed away.

If something began in the past and continues until the moment of speech, the present perfect is used.

The sentences below imply that the activity was begun in the past and has already ended.

When since is in the sentence, the present perfect is always used.

When might is introduced before the present perfect, have the main verb form is used even if the subject is third person singular.

There are no simple past forms for should, could and must. The past is expressed using a form of the present perfect.