Learn Hardcore Russian: В жару я читаю книгу в тени.

In Russian, when you express “during” a certain time or weather condition, you often use the pattern в + accusative. So в жару literally means “in hot weather,” and here жару is the accusative form of жара. It’s an idiomatic, fixed way to say “when it’s hot.”

Could we also say на жаре instead of в жару? If so, what’s the difference?

Yes, both в жару and на жаре are heard.

в жару emphasizes the time period “during hot weather.”
на жаре can feel more like “out in the heat” or “under the hot sun.”
In many contexts they’re interchangeable, but в жару is slightly more abstract/time-focused, while на жаре often has a physical/emphasis-on-location nuance.

Why is в тени in the prepositional case (тени) instead of the accusative like in в жару?

Here в expresses a static location (“in the shade”), so it takes the prepositional case: в + prepositional = location. That’s why тень → тени. In contrast, в + accusative is used for movement or, as above, idiomatic time expressions.

Could we change the word order? For example, “Я читаю книгу в тени в жару.” Is that acceptable?