אני שותה מיץ תפוזים בבוקר, אבל עכשיו אני רוצה לשפוך רק מים לכוס.

Because the sentence has two separate clauses:

• אני שותה מיץ תפוזים בבוקר
• אבל עכשיו אני רוצה לשפוך רק מים לכוס

In Hebrew, it is very normal to repeat the subject in a new clause, especially after אבל (but).

Also, in the present tense, Hebrew verbs do not show person clearly. רוצה tells you masculine/feminine singular, but not whether it means I want, you want, or he/she wants. So אני is needed to make the subject clear.

It can mean either one. Hebrew present tense often covers both:

• I drink
• I am drinking

The context tells you which meaning is intended.

In this sentence, בבוקר (in the morning) makes the first clause sound habitual: I drink orange juice in the morning.

In Hebrew present tense, there is usually no separate word for am / is / are before another verb.

So:

• אני שותה = I drink / I am drinking
• אני רוצה = I want

Hebrew does not need a separate present-tense form of to be here.

מיץ תפוזים is the normal Hebrew way to say orange juice. It is a very common noun + noun combination, sometimes called the construct state.

So:

• מיץ תפוזים = orange juice

You could say מיץ של תפוזים, but it sounds less natural here and more like juice of oranges.

Because את is used only before a definite direct object.

Here, both objects are indefinite or generic:

• מיץ תפוזים = orange juice
• מים = water

So there is no את.

Compare:

• אני שותה מיץ תפוזים = I drink orange juice
• אני שותה את מיץ התפוזים = I drink the orange juice

That is just how the word works in Hebrew. מים is the normal everyday word for water, even though it looks like a plural form.

It behaves like a mass noun, but grammatically it often takes plural agreement in other contexts, for example:

• מים קרים = cold water

So yes, מים means water, not waters in normal usage.

After רוצה (want), Hebrew normally uses the infinitive with ל־.

So:

• אני רוצה לשפוך = I want to pour
• אני רוצה לשתות = I want to drink
• אני רוצה ללכת = I want to go

That is the standard pattern.

לכוס means to the cup or, in natural English, into the cup. It shows direction or destination.

So:

• לכוס = into/to the cup
• בכוס = in the cup

With a verb like לשפוך (to pour), לכוס is natural because the water is moving into the cup.

If you want to be extra explicit, Hebrew can also say:

• לתוך הכוס = into the cup

רק means only.

In this sentence:

• לשפוך רק מים = to pour only water

So the idea is that the speaker wants water, not juice or anything else.

Its position matters. רק usually focuses the word or phrase that comes right after it.

Yes, but the meaning or emphasis changes.

For example:

• אני רוצה לשפוך רק מים לכוס = I want to pour only water into the cup
• אני רק רוצה לשפוך מים לכוס = I only want to pour water into the cup
• אני רוצה לשפוך מים רק לכוס = I want to pour water only into the cup

So רק is flexible, but where you place it affects what is being limited.

In normal unpointed Hebrew spelling, it can be either.

The forms שותה and רוצה are written the same for masculine singular and feminine singular, but they are pronounced differently:

• masculine: roughly shote, rotze
• feminine: roughly shota, rotza

So the written sentence fits either a male or female speaker.

Because אבל עכשיו naturally means but now, which creates a contrast with the earlier idea בבוקר (in the morning).

So the sentence contrasts:

• what usually happens in the morning
• what the speaker wants right now

That placement is very natural in Hebrew.