Learn Farsi: من نمی‌توانم دمپایی بپوشم چون دردم می‌آید.

This is an experiencer verb construction that literally means "my pain comes." It combines the noun درد (pain), the pronominal enclitic ـَم (my / to me), and the third-person singular verb می‌آید (comes). In Farsi, physical sensations are often expressed as things that happen to you, using a noun and a fixed third-person verb.

Why is the verb for wear, بپوشم, in the subjunctive form here?

Because it follows نمی‌توانم (I cannot). When you use a modal verb to express ability, the action that follows must be in the subjunctive mood, which is why it has the بـ prefix.

Why doesn't نمی‌توانم also take the subjunctive prefix?

The first verb in a modal chain reflects your actual, current reality, so it stays in the present indicative tense. Only the secondary action that you can or cannot do (بپوشم) takes the subjunctive form.

Why is دمپایی singular when the English translation says "slippers" in the plural?

In Farsi, when you talk about a generic category of an item rather than a specific set, you use the singular form. Since you mean "slippers in general," دمپایی remains singular.