tomodati to hamorou.

ハモろう is the volitional form of ハモる.

• ハモる = to sing/play in harmony, to harmonize
• ハモろう = let’s harmonize / shall we harmonize

So the ending -ろう here gives the sentence an inviting, suggestive feeling.

The dictionary form is ハモる.

This is a casual verb, and its volitional form changes like this:

• ハモる → ハモろう

This follows the pattern of many う-verbs:

• 歌う → 歌おう
• 帰る → 帰ろう
• ハモる → ハモろう

Here, と means with.

So:

• 友達と = with a friend / with friends

It marks the person you are doing the action together with.

In this sentence, 友達とハモろう means something like:

• Let’s harmonize with friends
• Let’s sing in harmony with a friend

depending on context.

Because 友達 is not the direct object here.

In this sentence:

• 友達と = with friends
• ハモろう = let’s harmonize

The friend is the companion in the action, not the thing being acted on, so Japanese uses と, not を.

It can mean either friend or friends.

Japanese nouns usually do not change form for singular vs. plural the way English nouns do. So:

• 友達 can be a friend
• 友達 can also be friends

You tell from context.

That is why this sentence could be understood as either:

• Let’s harmonize with a friend
• Let’s harmonize with friends

Because ハモる is a casual, somewhat slangy word that comes from ハーモニー or the idea of harmony.

Katakana is often used for:

• loanwords
• modern/colloquial expressions
• words with a trendy or spoken feel

So ハモる in katakana feels natural because it is not a traditional native Japanese verb in the same way as something like 歌う.

It is casual.

ハモろう is a plain-form invitation, so it sounds friendly and informal. You would use it with:

• friends
• classmates
• bandmates
• people you are close to

A more polite version would be:

• 友達とハモりましょう。

That sounds more formal or polite.

Mostly yes, but the nuance depends on context.

～ろう / ～よう often means:

• let’s ...
• shall we ...
• why don’t we ...

So ハモろう is not just a plain statement about the future. It is a suggestion or invitation that includes the speaker.

It has a friendly, encouraging tone.

Because Japanese often leaves out subjects when they are obvious from context.

In English, you would normally say:

• Let’s harmonize with friends.

In Japanese, the idea of we is already built into the volitional form ハモろう, so an explicit subject is usually unnecessary.

Usually, no.

In normal Japanese writing, this would usually be written without spaces:

• 友達とハモろう。

Spaces are sometimes added in learning materials to make the sentence easier to read, but standard Japanese does not normally separate words with spaces.

Yes.

The key verb is ハモる, which specifically refers to harmonizing—usually in music or singing. So this is not a general sentence like Let’s hang out with friends.

It strongly suggests something like:

• singing a harmony part
• matching voices musically
• performing together in harmony

So the musical context is important.

Yes. The pattern is:

• [person] と ハモろう

For example:

• みんなとハモろう。 = Let’s harmonize with everyone.
• 先輩とハモろう。 = Let’s harmonize with my senior.
• 彼とハモろう。 = Let’s harmonize with him.

So once you understand X と ハモろう, you can reuse it easily.