Верхний полок слишком горячий.

Because it describes полок, which is masculine singular nominative here. Russian adjectives must agree with the noun they modify in gender, number, and case.

So:

• верхний = masculine singular nominative
• if the noun were feminine, it would be верхняя
• if neuter, верхнее
• if plural, верхние

These are different nouns.

• полка usually means an ordinary shelf
• полок is a masculine noun that often refers to the wooden bench/platform in a bathhouse or sauna

So in this sentence, полок most naturally means the upper sauna bench, not a normal wall shelf.

It is in the nominative singular, because it is the subject of the sentence: the thing that is being described as too hot.

That is why the adjectives also appear in nominative singular masculine:

• верхний полок
• горячий

In Russian, the present-tense form of to be is usually omitted.

So Russian says:

• Верхний полок слишком горячий

where English needs:

• The upper bench is too hot

This is completely normal Russian syntax.

Слишком means too in the sense of more than is desirable, comfortable, or acceptable.

So:

• слишком горячий = too hot
• очень горячий = very hot

That is an important difference. Слишком suggests a problem; очень does not necessarily.

Because горячий is an adjective agreeing with the noun полок.

• горячий полок = a hot bench
• полок горячий = the bench is hot

By contrast, горячо is used differently. It can mean hot in an impersonal sense, for example:

• Здесь горячо = It’s hot here

So when you describe a specific noun, you normally use горячий, not горячо.

Most likely, yes. The word полок strongly suggests the bench/platform in a баня or sauna.

If you wanted to talk about an ordinary shelf in a room, полка would usually be the expected word.

Yes. Russian word order is fairly flexible, but the original sentence has a neutral, natural order:

• Верхний полок слишком горячий

If you change the order, the meaning stays similar, but the emphasis can change. For example:

• Слишком горячий верхний полок sounds more marked and emphasizes too hot

For a learner, the original order is the safest default.

Russian has no articles, so the sentence itself does not directly mark the or a/an.

English chooses the article from context. In a real-life situation, this would often be:

• The upper bench is too hot

because the speaker usually means a specific known bench.

Yes, if the context already makes it clear which bench you mean.

• Полок слишком горячий = The bench is too hot

But if there are multiple levels or benches, верхний helps specify that it is the upper one.

Yes. In basic, neutral Russian, adjectives usually come before the noun:

• верхний полок
• горячий камень
• холодная вода

Putting the adjective after the noun is possible in some special styles or set expressions, but for ordinary speech, before the noun is the normal pattern.