هذا يهمني.

هذا is the demonstrative this or this one. In this sentence, it stands by itself, so it means something like this thing / this matter / this.

It does not need a noun after it here. Arabic often uses demonstratives on their own this way.

يهمني breaks down into:

• يهمّ = matters / concerns
• ـني = me

So the verb already includes the object me as a suffix.

Because Arabic often attaches object pronouns directly to the verb.

So instead of using a separate word for me, Arabic uses ـني.

• يهمّ = it matters / it concerns
• يهمّني = it matters to me / it concerns me

English uses to me in one natural translation, but Arabic does not need a separate preposition here.

The verb itself already shows a third-person singular subject, so Arabic does not need a separate word for it.

In other words, يهمّ already means he/it matters depending on context.
Then هذا tells you what that it is referring to.

So the structure is roughly:

• هذا = this
• يهمني = matters to me

It is often analyzed as a sentence that begins with a topic/subject:

• هذا = the thing being talked about
• يهمني = what is being said about it

So the feel is close to:

• As for this, it matters to me
• This matters to me

Arabic often likes to begin with the thing under discussion.

Yes, you can.

Both are grammatical, but the emphasis is a little different:

• هذا يهمني = This matters to me

  • starts with this
  • gives هذا more topic emphasis

• يهمني هذا = This matters to me

  • starts with the verb
  • sounds a bit more like a straightforward verbal statement

In many contexts, both work, but هذا يهمني highlights this more strongly.

Yes, هذا is the singular masculine form of this.

If the thing referred to is feminine, use هذه instead:

• هذا يهمني = this (masc.) matters to me
• هذه تهمني = this (fem.) matters to me

So both the demonstrative and the verb form can change with gender.

Because هذا is a demonstrative that is mostly treated as a fixed form in MSA. Unlike many ordinary nouns, it does not usually show the normal visible case-ending changes learners expect.

So even though it is the subject here, you still just see هذا.

A rough pronunciation is:

HAA-dhaa ya-HUM-mu-nee

A few notes:

• هذا

  • haa is long
  • dh is like the th in this

• يهمني

  • the m is doubled
  • nee at the end is long

So do not pronounce it like a single m sound; hold it a little: ya-HUM-mu-nee.

Yes, very often.

هذا can point to:

• a thing
• a situation
• a statement
• an idea that was just mentioned

So هذا يهمني could mean this matters to me even if this refers to an abstract issue, not something you can physically point at.

They are close, but not exactly the same.

• هذا يهمني = This matters to me / This concerns me

  • more verbal
  • more about personal concern or relevance

• هذا مهم لي = This is important to me / for me

  • more adjectival
  • more about importance

In many situations they overlap, but يهمني often feels more like this is of concern/relevance to me, while مهم لي feels more like this is important to me.