One key to learning grammar is knowing the major parts of speech and their common errors. Nouns tell the names of people, places, things or ideas. Verbs give an action such as "jump" or a state of being like "is." Pronouns refer to nouns vaguely, as in "she."
A subject-verb agreement error occurs when the main noun of the sentence does not agree in number with the main verb. Both the noun and the verb should be either singular or plural. For instance, in "The boys is talking" the main noun "boys" is plural but the verb "is talking" is singular. To fix this error, change "is" to "are." Finding the subject is sometimes more difficult as in "The colony of ants march." The "colony," which is singular, is the main noun here, not ants, which is plural. Therefore, the verb "march" should be singular, "marches."
Verbs also change based on their tense, or when they occur. The three major tenses are past (-ed), present (base form or -s), and future (will). Verb tense should stay the same in sentences and paragraphs unless there is a change in when the action is happening. For example, "I jump on the bed and danced" is incorrect because "jump" is present tense and "danced" is past tense. Both verbs should keep a consistent tense.
Pronouns should refer clearly to a specific noun. In the sentence, "Sally and Sue finished her homework," the pronoun "her" is unclear because the reader doesn't know if "her" refers to "Sally" or "Sue." It should be changed to "their" to refer to both girls. Another pronoun problem is pronoun case. The pronouns "I," "he," "she," "they," and "we" are used when the person referred to is performing an action. The pronouns "me," "him," "her," "them," and "us" are used when the person is receiving the action and not performing it.