The document describes the present perfect tense in English grammar. It is used to describe actions that began in the past but continue in the present or actions that occurred at an unspecified time in the past. The present perfect is formed using have/has plus the past participle of the main verb. Examples of affirmative, negative, and interrogative forms are provided for the present perfect, as well as common time expressions used with it like already and just yet.

1. PRESENT PERFECT (HAVE/HAS + PAST PARTICIPLE of the main verb)
USE:
• To describe a situation which happened at an unspecified time in the past.
• To describe a situation which started in the past and is still continuing.
AFFIRMATIVE
I/you/we/they have written- ‘ve written
He/she/it has written - ’s written
NEGATIVE
I/you/we/they have not written- haven’t written
He/she/it has not written – hasn’t written
INTERROGATIVE
Have I/you/we/they written?
Has he/she/it written?
TIME EXPRESSIONS: already, just yet…

2. STUDENT’S BOOK pages 16,17
http://www.englishlearner.com/intermediate/index.shtml
http://www.ego4u.com/en/cram-up/tests/present-perfect-simple-3