Rule 1. A sentence starts with a capital letter and ends with a period/full stop, a question mark or an exclamation mark. Example :
1. My horse is very clever!.
2. Where do you live?

Rule 2. The order of a basic positive sentence is Subject-Verb-Object. (Negative and question sentences may have a different order.) Example :
1. Raju loves Geeta.
2. They were driving their car to Delhi.

Rule 3. Every sentence must have a subject and a verb. An object is optional. Note that an imperative sentence may have a verb only, but the subject is understood. Example :
1. Ram teaches English.
2. .Stop! (i.e. You stop!)

Rule 4. The subject and verb must agree in number, that is a singular subject needs a singular verb and a plural subject needs a plural verb.Example :
1. John works in London.
2. That monk eats once a day.