The subject and verb of a sentence must agree in number. In other words, if the subject is singular, then the verb must be in singular form. A plural subject requires a plural verb.
With regular verbs, a single subject takes an “-s” at the end of the verb, while plural subjects do not.
This report recommends controlling mosquito populations. (subject = This report)
We recommend controlling mosquito populations. (subject = We)
English has a variety of irregular verbs, notably the forms of ‘to be’, and many help verbs.
Puzzles falling into this category are very challenging for humans. (subject = Puzzles)
This category of puzzles is very challenging for humans. (subject = This category)
The number of a verb reflects the number of the subject of the sentence or clause, not the most recent noun. The situation can be confusing when the most recent noun is in plural but the subject is singular, or when the subject is plural but the most recent noun is singular. In a longer sentence, it can be difficult to distinguish which noun is the subject, especially if several nouns of a different number occur between the subject and the verb.
Currently, the Tiger mosquito, Aedes albopictus, a tropic and subtropic species that is a vector for around 20 different arboviruses that can cause diseases, is found in southern Germany. (subject = the tiger mosquito)