EFL shorts has today got a good exercise today relating to the correspondence between the subject and a verb in a sentence. After this definition there is a small exercise which you can find here.
This simply means that the verb (or auxiliary verb) must pay attention to whether the subject is singular or plural. Look at this sentence: ‘The teacher was standing at the window’. In this sentence, the subject is ‘the teacher’. There is just one teacher, so the subject is singular. Because the subject is singular, the verb must ‘match’ it. Therefore, the correct (auxiliary) verb form is ‘was’ (and not ‘were‘).