i hve problem with suject verb agreement. the students ask me why do we put s or es alongwith the verb in case of a sigular subject in present indefinite tense. they understand that we need it for singular subjects. but they always ask why do we need it. plz help me in this reagrd.
Interesting question with an even more interesting answer (I doubt your students will like it Emotion: smile )

To tell the truth:

We don't actually "need" the 3rd person singular -s anymore, we could even get along without it very well.

The -s is kind of a hangover from ancient times: In Old-English and even in Middle-English, every person had its own ending for every verb, e.g.

I have
thou hast
he hath...

even in the past tense, the forms had special endings for each person, eg:

I had
thou hadst...

This is because the personal pronouns are "quite" new in Germanic languages:
In former times (esp. Old-English and earlier) personal pronouns were not needed as everything important could be expressed by the ending of the verb.
Have a look at Latin or even the Spanish or the Italian language, where this old scheme is preserved until today. They have developed personal pronouns though, but they're hardly used as the old ending paradigm still works very well.

In Germanic languages, these endings became less important as the newer method of using a personal pronoun seemed to be much more effective as it was much easier to keep in mind these pronouns than dealing with all the special endings, also because there were a lot irregular forms.
Have a look here at Swedish or Danish for example, where all endings for every person have gone lost already

The -s for 3rd person singular in current English is "just" a hangover from this ancient paradigm which, as far as I know does tend to be lost though in some dialects: Some speakers of special dialects already say "he go, she write, he want" instead of "he goes, she writes, he wants".

I hope I could help you out - and your students will be satisfied by thisEmotion: smile
first and foremost that's the rule set by standards.
 Pemmican's reply was promoted to an answer.