Uses of the past tenses

The simple past tense Form: Subject + past tense of the verb I worked. She sang. He played. Mother cooked. The simple past tense is used to talk about an action completed in the past. It is often used with adverbs or adverb phrases of past time.

Practical Grammar 1

Each level of Practical Grammar has 100 units, divided into modules of five units. Each module examines a particular area of grammar. The grammar is set in short, everyday conversations or texts, showing the language in natural situations which students will find engaging and can relate to. Students then examine the form, meaning and use of the language before practicing it in a variety of activity types.
Introduction, Appendices, Answer Key with tapescripts, Index added Thanks to  anonymous

Introductory it

When the subject is an infinitive phrase, the sentence often begins with it. Instead of saying ‘To find fault with others is easy’, we say, ‘It is easy to find fault with others’. More examples are given below. It is easy to learn English. (More natural than ‘To learn English is easy’).

Present perfect tense

Form:

has/have + past participle form of the verb He has written a letter. They have arrived. It has stopped raining. The present perfect tense is used to talk about completed activities in the immediate past. It is often used with the adverb of time just. I have just finished my work.

Present continuous tense

The present continuous tense is used to talk about an action going on at the time of speaking...