Learn Farsi: با اینکه آنها وسایل زیادی را انباشته‌اند، تنها از ضروریات استفاده می‌کنند. - Even though they have hoarded many items, they solely use the essentials.

In this sentence, tanhâ is used for emphatic restriction. Paired with the affirmative verb estefâde mikonand, it emphasizes that they use exclusively the essentials, nothing else. You could use faqat (only), but tanhâ (or serfan) sounds slightly more formal and places a stronger, more emphatic contrast against the vast amount of things they hoarded.

Why is the definite object marker râ used after vasâyel-e ziyâdi when 'many items' sounds indefinite in English?

This is a common trap! While the suffix -i normally marks an indefinite noun, when attached to ziyâd it simply forms the quantifier ziyâdi (many / a lot of). Because the speaker is referring to the specific, established mass of items that these people have hoarded, râ is used to mark that known collection as the direct object.

What kind of plural form is used in the word zaruriyât (essentials)?

It uses the Arabic plural suffix -ât. Here, it is attached to the adjective zaruri (necessary) to turn it into the plural noun 'essentials' or 'necessary things.' You will see this -ât suffix frequently in formal, abstract, or categorized vocabulary at the B2 level.

Why do the two clauses use different verb tenses?

The first verb, anbâshte-and (they have hoarded), is in the present perfect tense because the hoarding happened in the past but the result (the pile of hoarded items) is still relevant now. The second verb, (they use), is in the present tense because using solely the essentials describes their ongoing, regular habit.