Learn Farsi: لباسی و دستکشی در حیاط هستند.

The suffix ی is the indefinite marker. It acts like "a" or "an" in English. So لباس (clothes or dress) becomes لباسی (a dress), and دستکش (glove) becomes دستکشی (a glove).

Do I have to put the indefinite marker ی on both things, or could I just say لباس و دستکشی?

You need to attach it to both! In Farsi, when you have multiple indefinite items connected by "and" (و), each noun gets its own indefinite marker ی. So it must be لباسی و دستکشی (a dress and a glove).

Since "dress" and "glove" are inanimate objects, why do we use the plural verb هستند (they are)?

Great catch! While a simple plural inanimate noun (like "gloves") often takes a singular verb in Farsi, when you have two or more distinct subjects joined by و (and), they form a plural compound subject. Because there are two separate items here, they take the plural verb هستند.

How is the word "and" (و) pronounced in this sentence?

In spoken Farsi, the conjunction و is almost always pronounced as a quick "o" sound rather than "va". Between the two indefinite words, you will hear it flow together smoothly as "lebas-i o dastkesh-i".