Rules for constructing sentences are that a sentence must have a subject and a predicate, the sentence needs to express a complete thought, and the subject and predicate must agree with one another. Sentences must be punctuated correctly to prevent run-ons.
Sentences must have a noun or pronoun, even if it is merely understood, and a verb in order to be correct. The subject, which is a noun or pronoun, and the predicate, which is a verb, must agree in number. If the subject is singular, the verb must be in singular form, and if the verb is plural, the verb must be plural.
A sentence must express a complete thought. For example, "I left" is a complete sentence, but "When I left" is not because it does not express a complete thought.
Punctuation is important is sentence construction. For example, "She went to the store she needed milk she forgot the milk she bought other things" is a run-on sentence. It should read "She went to the store because she needed milk. She forgot the milk, but she bought other things."