Learn Farsi: این سنگین‌ترین کامیونی است که یک نفر بتواند با دندان بکشد! - This is the heaviest truck that a person can pull with their teeth!

Because the relative clause (که یک نفر...) modifies a superlative adjective (سنگین‌ترین). In Farsi, when you describe the 'heaviest', 'best', or 'worst' thing that someone might do, you use the subjunctive mood to show it is a hypothetical limit or possibility rather than a plain fact.

Why is there an ی at the end of کامیونی?

This is the restrictive ی. When a noun is modified by a relative clause starting with که (that), especially after a superlative, we often add this ی to the noun to link it tightly to the description. It signals that we are pinpointing a specific instance: 'the particular truck that...'.

Both بتواند (can) and بکشد (pull) have the subjunctive بـ prefix here. Do you normally put two subjunctive verbs in a row?

Yes! The verb توانستن (to be able to) always forces the action verb that follows it into the subjunctive, which gives us بکشد. At the same time, توانستن itself must be in the subjunctive (بتواند) because it is the main verb inside the relative clause describing a superlative. So they both get the prefix!

The English translation says 'with their teeth', but دندان is singular. Shouldn't it be plural?

In Farsi, when you talk about using a body part as a general tool or instrument (like pulling 'with teeth' or seeing 'with the eye'), it is most natural to leave it in the singular form. Adding a plural marker here would sound slightly unnatural.