האירוע מתחיל מחר.

It is the Hebrew definite article, equivalent to the in English.

• אירוע = event
• האירוע = the event

In Hebrew, ה attaches directly to the beginning of the noun.

A rough pronunciation is:

ha-eiru'a matchil machar

A few tips:

• מתחיל and מחר use the Hebrew ח sound, which is like the ch in Scottish loch or German Bach, not the ch in chair.
• מתחיל is roughly mat-CHIL
• מחר is roughly ma-CHAR

Because האירוע is a masculine singular noun, and the present-tense verb form agrees with it.

• masculine singular: מתחיל
• feminine singular: מתחילה

So:

• האירוע מתחיל מחר = The event starts tomorrow
• המסיבה מתחילה מחר = The party starts tomorrow

You mostly have to learn the gender of Hebrew nouns as part of the vocabulary item.

Some nouns have endings that strongly suggest gender, but אירוע does not have a typical feminine ending like -ה or -ת, and it is simply a masculine noun.

So the correct agreement is:

• האירוע מתחיל not
• האירוע מתחילה

Hebrew often uses the present tense for a planned or scheduled future event, just as English does in sentences like The event starts tomorrow.

The word מחר makes the future time clear, so Hebrew does not need a future-tense verb here.

This sentence sounds very natural for something scheduled.

Yes, absolutely.

Both are correct:

• האירוע מתחיל מחר
• האירוע יתחיל מחר

A useful way to think about the difference:

• מתחיל מחר often sounds like a scheduled or arranged event
• יתחיל מחר is a straightforward future statement

In many real situations, the difference is small.

No. Hebrew usually does not use a present-tense form of to be the way English does.

Also, in this sentence, מתחיל is already a full verb meaning starts / is starting. So the sentence is complete without any extra word.

The dictionary form is להתחיל, meaning to start or to begin.

מתחיל is the masculine singular present form.

Related forms are:

• מתחיל = masculine singular
• מתחילה = feminine singular
• מתחילים = masculine plural / mixed plural
• מתחילות = feminine plural

Because מחר by itself already means tomorrow and works as a time adverb.

English sometimes uses extra words in time expressions, but Hebrew often does not. So:

• מתחיל מחר = starts tomorrow

No preposition is needed.

The order here is the most neutral and common one:

• האירוע = subject
• מתחיל = verb
• מחר = time expression

So האירוע מתחיל מחר is the natural default.

Hebrew word order can be flexible, though. For emphasis, you could move מחר to the front:

• מחר האירוע מתחיל

That is possible, but it sounds more marked or emphatic. For a basic sentence, the original order is best.

In this sentence, it can cover the meaning that English expresses with either starts or, in some contexts, is starting.

Hebrew does not always separate those two ideas the same way English does. Here, because of מחר, the most natural English translation is usually starts tomorrow.