In order for a sentence to be grammatically correct, the subject and verb must both be singular or plural. In other words, the subject and verb must agree with one another in their tense. If the subject is in plural form, the verb should also be in plur al form (and vice versa).
They used the Nile's floods to their advantage. Every time the Nile flooded, it deposited silt in the soil, which made the soil great for growing crops.
