Rama,
Your English seems okay. You have used verbs quite successfully in your post. Do you not have a specific question?
The base form of the English verb is the unconjugated, non-finite form:
To work.
In this form it does not exhibit tense.
If it is a regular verb you form the past tense by adding the suffix ed.
Worked.
If it is irregular, the past form does not follow this rule, and it must be learnt as a new word.
To buy = bought.
The present tense is very simple to use as it is the unconjugated form of the verb. It only changes when it must agree with a singular subject in the 3rd person. (We add an s to the verb)
I/You/We/they buy...
John/He/She/It buys...
We form the continuous tense by adding ing to the verb in its unconjugated form. The tense is determined by a form of to be which preceeds the ing form of the verb.
I am/was working.
Verbs are not inflected to form the future tense. We use present tenses and auxilary verbs to form the future tense.
I leave at 9:00 (present simple).
I am working later (present continuous).
I will/shall decide what I want to do next week (will/shall auxilaries).
I am going to buy a new car (be going auxilary).
This is a very, very rough outline which leaves much to be said. A lot of what I have written is not definitive: it does not apply universally. There are exceptions.
If you have a specific question regarding a particular use of a tense in context, you will receive a much more useful answer.