1. The tense and verb must agree:
Present tense: I am walking in the park with my dog.
Past tense: Yesterday I walked in the park with my dog.
Future tense: Tomorrow I will walk in the park with my dog.
2. Verbs need to agree with their subjects in number (singular or plural) and in person (first, second, or third).
3. Compound subjects are plural, and their verbs should agree.
Example: A pen, a diary, and a map were issued to each student.
Explanation: the subject of this sentence is made up of three objects (pen, diary and map) and so is plural. This means the verb (were) must also be plural.
4. When the verb comes before the subject, make the verb agree with the subject, not with a noun that happens to go before it.
Example: Behind the fence grow gardens filled with roses.
Explanation: The subject is gardens; it is plural, so the verb must be grow NOT grows.