When the subject of a sentence isn't doing something, the verb is passive. On the other hand, a sentence is active when the subject performs the verb (action). For example, in this sentence the verb ...

As a linguist, I’ve lost count of how many times I have been asked what I think of the various language-learning apps. The truth is that I don’t use them. But of late I have been watching my daughter, ...

Subject-verb agreement means that your verb must be conjugated, or changed, to fit (or agree) with the subject. Subjects can be singular or plural. Think of singular and plural as mathematical ...