Sentence problems: agreement
Noun-pronoun agreement
Pronouns must agree in number with the noun they refer back to:
Incorrect:
(1a) It is a device used for storing and supplying electricity, and they are becoming increasingly important because of the rapid electrification of our society.
Correct:
(1b) They are devices used for storing and supplying electricity, and they are becoming increasingly important because of the rapid electrification of our society.
Subject-verb agreement
A verb must agree in number with its subject. This is important in the whole sentence, not only for the first verb.
Incorrect:
(2a) The building is constructed of concrete and are four stories high.
Correct:
(2b) The building is constructed of concrete and is four stories high.
Be careful if the subject is separated from the verb by another noun with a different number. To get it right, you must know which noun is the head noun, or the most important noun. In example (3), it is the use of the materials that is encouraged, not the materials themselves.
Incorrect:
(3a) The use of these sustainable materials are encouraged.
Correct:
(3b) The use of these sustainable materials is encouraged.