kawa no nagare ha hayai.

川の流れ means the flow/current of a river.

• 川 = river
• の = a linking particle, often like of
• 流れ = flow, stream, current

So 川の流れ is literally something like the river’s flow or the flow of the river.

の connects nouns.

In this sentence, it shows that 流れ belongs to or is associated with 川:

• 川 → river
• 流れ → flow/current
• 川の流れ → the river’s current / the flow of the river

This is one of the most common uses of の in Japanese: noun + の + noun.

は marks the topic of the sentence.

Here, the topic is 川の流れ:

• 川の流れは = As for the river’s current / The river’s current...

Then the sentence says something about that topic:

• 速い = is fast

So the structure is:

• 川の流れ = the river’s current
• は = topic marker
• 速い = fast

A natural way to understand it is: As for the river’s current, it is fast.

This is a special feature of Japanese.

When は is used as the topic particle, it is pronounced wa, not ha.

So:

• 川の流れは速い is read:
• かわ の ながれ はやい with the は particle pronounced wa

This only happens in certain grammatical uses. As a normal syllable in a word, は is usually pronounced ha.

In Japanese, sentences with i-adjectives often do not need a separate verb like is.

• 速い already functions as the predicate
• so 速い can mean is fast

That means:

• 川の流れは速い literally works as
• As for the river’s current, fast

but in natural English, that becomes:

• The river’s current is fast.

This is very normal in Japanese.

速い is an i-adjective.

That means it ends in -い and can directly describe something or act as the main predicate of a sentence.

Examples:

• 速い川の流れ = a fast river current
• 川の流れは速い = the river’s current is fast

Because it is an i-adjective, you do not need です for it to be grammatical in plain style.

Yes, both are pronounced hayai, but they mean different kinds of fast/early.

• 速い = fast in speed
• 早い = early in time

Since a river current has to do with speed, 速い is the correct kanji here.

So:

• 流れが速い = the current is fast
• 時間が早い would be wrong for early
• 時間が早い should usually be 時間が早い? Actually for time, you'd normally use expressions like 早い時間 or 来るのが早い

The key point is:

• use 速い for movement/speed
• use 早い for time/earliness

Yes, 川の流れが速い is also possible, but the nuance changes.

• は marks the topic and often sounds more general, contrastive, or descriptive
• が marks the subject more directly and can feel more like identifying what is fast

Compare:

• 川の流れは速い。
  As for the river’s current, it is fast.
  This sounds like a general statement about it.

• 川の流れが速い。
  The river’s current is fast. / It’s the river’s current that is fast.
  This can sound more like new information or emphasis on what is fast.

Both are natural, but は is very common when stating a general fact.

Because this sentence is in plain style.

• 川の流れは速い。 = plain style
• 川の流れは速いです。 = polite style

Both are correct.

Use:

• 速い in casual writing, dictionary style, or plain speech
• 速いです in polite conversation

The meaning stays basically the same.

It is pronounced:

かわ の ながれ は はやい

Remember:

• 川 = かわ (kawa)
• 流れ = ながれ (nagare)
• the particle は is pronounced wa

So if written in romaji, it would be:

kawa no nagare wa hayai

Yes. 流れ can mean flow, current, or even a figurative course/trend depending on context.

Examples:

• 川の流れ = the flow/current of a river
• 時代の流れ = the flow of the times
• 話の流れ = the flow of the conversation

In this sentence, because of 川, it clearly means the physical current of the river.

Japanese does not use articles like English does.

So 川, 流れ, and 川の流れ do not automatically tell you a river, the river, a current, or the current. The exact meaning depends on context.

That is why the sentence could be translated in slightly different ways, such as:

• The river’s current is fast.
• A river’s current is fast.
• The flow of the river is fast.

In real use, context usually makes the intended meaning clear.

Yes, in natural English that can work, but it is not the most literal translation.

The Japanese sentence is talking about the river’s flow/current as a noun phrase:

• 川の流れ = the river’s flow/current

So the most direct translation is:

• The river’s current is fast.

But in smoother English, people might also say:

• The river flows fast.
• The river is fast-flowing.

Those are natural English equivalents, even though the Japanese structure is noun-based.