אני חושבת שהוא ישראלי, כי הוא מדבר עברית.

In Hebrew present tense, verbs do not change for person the way they do in English. Instead, they mainly change for gender and number.

So חושב / חושבת / חושבים / חושבות can mean:

• I think
• you think
• he thinks
• she thinks

The pronoun tells you who the subject is. Here, אני tells you the meaning is I think.

The ending -ת marks feminine singular in the present tense.

So:

• אני חושבת = said by a female speaker
• אני חושב = said by a male speaker

Hebrew often marks gender in the present tense, even in the first person, which is something English does not do.

שהוא is made of:

• ש־ = that
• הוא = he

So literally, אני חושבת שהוא ישראלי is something like:

I think that he [is] Israeli

In modern Hebrew, ש־ is very commonly attached directly to the next word, so ש + הוא becomes שהוא.

In Hebrew, the verb to be is usually not expressed in the present tense.

So:

• הוא ישראלי = he is Israeli
• literally: he Israeli

This is completely normal in Hebrew.
But in the past or future, forms of to be do appear.

For example:

• הוא היה ישראלי = he was Israeli
• הוא יהיה ישראלי = he will be Israeli

Because it describes הוא, which is he. Hebrew adjectives agree with the noun or pronoun they describe in gender and number.

So:

• הוא ישראלי = he is Israeli
• היא ישראלית = she is Israeli

The masculine singular form is ישראלי.
The feminine singular form is ישראלית.

For the same reason: it agrees with הוא.

In the present tense, Hebrew verbs agree with the subject in gender and number.

So:

• הוא מדבר עברית = he speaks Hebrew
• היא מדברת עברית = she speaks Hebrew

Here, מדבר is the masculine singular form because the subject is he.

Hebrew has a word for the — the prefix ה־ — but it does not have an indefinite article like a/an.

So:

• הוא ישראלי can mean he is Israeli or he is an Israeli, depending on context.
• הוא הישראלי would mean he is the Israeli, which is a different meaning.

This is very common in Hebrew: no separate word for a or an.

Here, כי means because.

It introduces the reason:

• אני חושבת שהוא ישראלי = the statement
• כי הוא מדבר עברית = the reason

So the structure is: I think he is Israeli, because he speaks Hebrew.

In other contexts, כי can sometimes mean that, but in this sentence because is the natural meaning.

After לדבר (to speak), Hebrew can use the language name directly without a preposition.

So:

• מדבר עברית = speaks Hebrew

This is the most common and natural pattern with languages.

You may also sometimes hear forms with ב־ in certain contexts, but for basic statements like this, מדבר עברית is standard and very common.

If the speaker were male:

• אני חושב שהוא ישראלי, כי הוא מדבר עברית.

If the person being talked about were female:

• אני חושבת שהיא ישראלית, כי היא מדברת עברית.

If both changed:

• אני חושב שהיא ישראלית, כי היא מדברת עברית.

This is a good example of how Hebrew keeps track of gender in several places:

• the speaker: חושב / חושבת
• the subject being described: הוא / היא
• the adjective: ישראלי / ישראלית
• the present-tense verb: מדבר / מדברת