To write a grammatically correct sentence, find the subject and the verb, and ensure that the verb is not left hanging. The subject and the verb must agree with each other, and any verbs in the sentence must be parallel.
The subject of a sentence is the person or thing performing the action expressed by the verb, and the verb is the action the subject is completing or the state it's in. For example, in "The dog ran down the street," the subject is "the dog" and the verb is "ran." Some verbs require additional information to complete the meaning, such as in the sentence "He ripped the paper." To simply say "he ripped" is not grammatical because it requires the direct object "the paper" to complete its meaning.
The subject and verb of a sentence must also agree with each other in number. That is, if the subject is singular, the verb must also be singular. For example, "the ladies" is a plural noun phrase, so it must agree with a plural verb form, as in "The ladies sing beautifully." To say "The ladies sings beautifully" is incorrect.
Similarly, all verbs in a sentence must be parallel with each other in number and tense. For example, "Bobby likes to sing, dance and playing piano" is incorrect because "playing" is in a different form than "sing" and "dance." The correct form is "Bobby likes to sing, dance and play piano."