Learn Turkish: Mandalina kışın çok bol olur. - Tangerines are very plentiful in winter.

In Turkish, when talking generally about a whole category or type of thing, we usually use the singular form. Because we mean "tangerines in general," mandalina stays singular.

Why is "in winter" written as kışın instead of using the locative case like kışta?

For seasons and certain times of day, Turkish uses a special ending -ın / -in to mean "during" or "in". So kışın means "in winter" and yazın means "in summer." This is a fixed expression that sounds much more natural than kışta.

What exactly does bol mean?

bol means "plentiful," "abundant," or "copious." You will also often see it used to describe clothing that is "loose" or "baggy." Here, çok bol emphasizes that tangerines are very abundant.

Why does the sentence end with olur instead of just boldur?

olur is the aorist (general time) form of olmak (to be/become). Using olur shows that this is a general truth, habit, or natural recurring fact. It implies that tangerines "routinely become" or "generally are" plentiful every winter.