gensen ha atui.

It is read:

げんせん は あつい。
Romanization: gensen wa atsui.

A small but important point: the particle は is written ha but pronounced wa when it marks the topic.

Here, 源泉 means the source of a hot spring or the spring source.

In other contexts, 源泉 can also mean a more abstract source or origin, such as the source of funds or the source of energy. But with 熱い, the physical meaning is the natural one: the spring source is hot.

は marks the topic of the sentence.

So 源泉は熱い means something like:

• As for the spring source, it is hot.
• The spring source is hot.

Japanese often works with a topic-comment structure:

• 源泉は = as for the spring source
• 熱い = is hot

This does not always match English word-for-word, but that is the basic grammar pattern.

This is a special feature of a few Japanese particles.

When は is used as a particle, it is pronounced わ.

So:

• 源泉は = げんせんわ
• not げんせんは

This is something you simply have to memorize. The spelling stays は, but the pronunciation becomes wa.

Yes, you could, but the nuance changes.

• 源泉は熱い = As for the spring source, it is hot.
  This sounds more like a general statement, description, or contrastive topic.

• 源泉が熱い = The spring source is hot.
  This can sound more like you are identifying it specifically, emphasizing that it is the thing that is hot.

Very roughly:

• は = topic / contrast / general statement
• が = subject focus / identification / emphasis

Both can be correct depending on context.

Both can be translated as hot, but they are used differently.

• 熱い（あつい）: hot to the touch, hot objects, hot liquids, hot things physically containing heat
• 暑い（あつい）: hot weather, hot air, a hot day, a hot room because of temperature

Since a spring source or its water is physically hot, 熱い is the correct kanji here.

Examples:

• お湯が熱い。 = The hot water is hot.
• 今日は暑い。 = It’s hot today.

熱い is an い-adjective.

That means it can end a sentence by itself in plain form:

• 源泉は熱い。 = The spring source is hot.

You do not need a separate word for is in casual/plain Japanese here.

Some related forms:

• 熱くない = not hot
• 熱かった = was hot
• 熱くなかった = was not hot

Because this sentence is in plain style.

• 源泉は熱い。 = plain / casual / neutral written style
• 源泉は熱いです。 = polite style

Both are grammatically correct.

Japanese often omits です in plain speech and in simple written statements. If you are speaking politely, 熱いです is more appropriate.

Yes, it is a complete sentence.

Japanese does not need an explicit word like English is in this kind of sentence. The adjective 熱い works as the predicate by itself.

So the structure is:

• 源泉は = topic
• 熱い = predicate

That is enough to form a full sentence.

No. Japanese is normally written without spaces.

So the normal written sentence is:

源泉は熱い。

Spaces are often added in beginner materials to make the parts easier to see, but native Japanese writing usually does not use them.

源泉 can mean more than just a hot spring source.

It can also be used in broader or abstract senses, such as:

• the source of money
• the source of power
• the origin of something

However, in 源泉は熱い, the adjective 熱い strongly suggests a literal, physical source that is hot, so learners will normally understand it as a hot spring source.

A very literal breakdown is:

• 源泉 = spring source / source
• は = as for
• 熱い = hot

So literally:

As for the spring source, it is hot.

A more natural English translation is simply:

The spring source is hot.