תשמור את הקבלה, כי אולי נרצה להחליף את החולצה מחר.

A natural pronunciation is:

Tishmor et ha-kabala, ki ulai nirtze lehakhlif et ha-khultza makhar.

A few sound notes:

• ח in החולצה and מחר is the throaty kh sound, like in Scottish loch.
• Stress is usually near the end: kabaLA, nirTZE, lehakhLIF, khulTZA, maKHAR.

In modern Hebrew, speakers often use the future form to give an instruction in a slightly softer, more natural way.

So:

• שמור את הקבלה = a direct imperative
• תשמור את הקבלה = a very common conversational way to say the same thing, often sounding a bit less abrupt

It can feel like keep the receipt or make sure to keep the receipt.

Because תשמור can actually mean either:

• you will keep addressed to one male
• she will keep

Hebrew often leaves this kind of ambiguity in the written form, and context tells you which meaning is intended. Here, the sentence is clearly speaking to someone directly, so it is understood as you.

If you were speaking to one woman, you would normally say תשמרי.

Yes. Here תשמור is addressed to one male.

Other common forms would be:

• to one female: תשמרי את הקבלה
• to more than one person: תשמרו את הקבלה

So this sentence, as written, is most naturally directed at one male listener.

Here את is the direct object marker. It does not have a separate English translation.

Hebrew uses את before a definite direct object, usually a noun with ה־.

So:

• את הקבלה
• את החולצה

Both are specific things, so את is used.

Also, do not confuse this את with אַתְ, which means you addressed to a woman. In unpointed Hebrew they are written the same way, but they are different words.

The prefix ה־ means the.

So:

• קבלה = a receipt
• הקבלה = the receipt
• חולצה = a shirt
• החולצה = the shirt

In this sentence, both nouns are definite because they refer to specific things already understood from the situation: the receipt from the purchase, and the shirt that was bought.

• כי = because
• אולי = maybe / perhaps

Together, כי אולי means something like because maybe or more naturally because we might...

This combination is completely normal in Hebrew. The speaker is giving a reason for keeping the receipt: there is a possible future need.

נרצה comes from לרצות, meaning to want.

It is future tense because the possible wanting is in the future. Hebrew often expresses English ideas like may want or might want by using:

• אולי
  • future tense

So:

• אולי נרצה = maybe we will want / we might want

Hebrew usually does not need a separate word exactly like English might here.

להחליף means to replace, exchange, switch.

In a shopping context, להחליף את החולצה usually means to exchange the shirt, for example for:

• another size
• another color
• a different one

If you wanted to say return the shirt for money back, Hebrew would more often use להחזיר.

The ל־ is the normal Hebrew marker for the infinitive, similar to English to.

So:

• להחליף = to exchange
• לשמור = to keep
• לרצות = to want

This is why dictionary forms of Hebrew verbs often begin with ל־.

Yes, it could go elsewhere. Hebrew word order is fairly flexible.

The version here, with מחר at the end, sounds natural and neutral:

• כי אולי נרצה להחליף את החולצה מחר

You could also say:

• כי אולי מחר נרצה להחליף את החולצה
• כי אולי נרצה מחר להחליף את החולצה

All of these are grammatical. The difference is mostly about rhythm and emphasis, not basic meaning.