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 ...

Even so, they are still perceived individually or as a totality, so the singular verb form is still maintained ... etc. In your example, Ihsan, as there is no relative clause following all ...

What is it about the subjunctive form that makes both native and nonnative users of English intimidated and prone to error ...

In this case, be careful about whether you are taking in the singular or plural meaning and make sure to change the verb. Other examples include: economics, sheep, politics, headquarters ...