Final answer:
The subject and verb in a sentence must agree in number, whether they are singular or plural.
Step-by-step explanation:
The subject of a sentence names something and the predicate contains the verb, which expresses an action, an occurrence, or a state of being. The subject and verb must agree in number, meaning they must both be singular or both be plural. For example, 'The dog chases the ball' and 'The dogs chase the ball' have subject-verb agreement.
In compound subjects joined by 'and', the verb is plural. In 'Yoga and meditation are effective activities for relieving stress', 'are' is a plural verb because 'yoga' and 'meditation' are two subjects joined by 'and'.
When words come between the subject and the verb, the verb must still agree with the subject. For example, 'The cost of the flights is prohibitive' and 'A box of invitations with stamps and return addresses was on the desk' have subject-verb agreement even though there are additional phrases between the subject and verb.