onaka ga suitara, konbini de onigiri wo kaimasu.

In お腹がすく, the thing that “becomes hungry” is marked with が. This is the normal pattern for this expression.

• お腹がすく = to get hungry / for one’s stomach to become empty
• So お腹がすいたら literally works like if/when your stomach gets empty

You can use は in some contexts, but it changes the nuance:

• お腹がすいたら = if/when you get hungry
• お腹はすいたら = sounds marked or contrastive, as if comparing the stomach with something else

For a basic sentence like this, が is the natural choice.

すいたら comes from the verb 空く（すく）, which means to become empty. In the expression お腹がすく, it means to get hungry.

The form breaks down like this:

• 空く → dictionary form
• 空いた → past form
• 空いたら → if/when it becomes empty / if/when you get hungry

So:

• お腹がすいたら = if/when you get hungry

This is a very common grammar pattern:

• verb in た-form
  • ら = if/when

It can feel like either if or when, depending on context.

In this sentence, お腹がすいたら、コンビニでおにぎりを買います, it often sounds like:

• When I get hungry, I buy a rice ball at the convenience store
• or If I get hungry, I buy a rice ball at the convenience store

Why both are possible:

• たら introduces a condition
• but in everyday Japanese, that condition can be a realistic, expected situation, so English may use when

A good simple way to understand it is:

• たら = if/when after that happens

で marks the place where an action happens.

Here, the action is 買います (buy), and that action happens at the convenience store.

• コンビニで買います = buy at the convenience store

This is different from に:

• で = location of an action
• に = destination, target, or place of existence in many cases

So:

• コンビニでおにぎりを買います = I buy a rice ball at the convenience store

That is why で is correct here.

を marks the direct object of the verb—the thing that is being acted on.

In this sentence, the verb is 買います (buy), and the thing being bought is おにぎり.

So:

• おにぎりを買います = buy a rice ball

This is one of the most basic uses of を in Japanese.

Japanese non-past can express both:

• future actions
• habitual actions
• general tendencies

So 買います here could mean:

• I will buy a rice ball at the convenience store when I get hungry
• When I get hungry, I buy a rice ball at the convenience store

Without more context, both are possible. In many textbook-style sentences like this, it often sounds like a habitual action or a general plan.

Japanese does not separate present and future as strictly as English does.

Japanese often leaves out subjects when they are understood from context.

So in this sentence, the subject is omitted:

• お腹がすいたら、コンビニでおにぎりを買います。
• implied: I buy a rice ball at the convenience store when I get hungry

In natural Japanese, saying 私は every time would often sound unnecessary. If the speaker is obviously talking about themselves, Japanese usually just omits it.

So this sentence is completely normal without 私は.

お腹（おなか） is the normal, polite/common everyday word for stomach or belly.

• お腹 = neutral and common in conversation
• 腹（はら） = can sound rougher, more direct, or more literary depending on context

The お here is an honorific/polite prefix that often makes a word sound softer or more natural in daily speech.

So for a beginner sentence like this, お腹がすく is the standard expression.

Yes. コンビニ is a very common Japanese word borrowed from English convenience store.

It is an abbreviation:

• コンビニエンスストア = convenience store
• コンビニ = shortened everyday form

Japanese uses a lot of shortened loanwords like this, and コンビニ is the normal word people actually use in conversation.

No, that would not be correct in this sentence.

Use で, not に, because you are describing the place where the action of buying happens.

• コンビニでおにぎりを買います = correct

If you use に, it usually suggests a destination or target, not the location of the action itself.

For example:

• コンビニに行きます = go to the convenience store
• コンビニで買います = buy at the convenience store

So:

• 行く takes に
• 買う at a location takes で

Yes, Japanese word order is more flexible than English, as long as the particles stay correct.

The original sentence is:

• お腹がすいたら、コンビニでおにぎりを買います。

You could also say:

• お腹がすいたら、おにぎりをコンビニで買います。

Both are understandable. However, the original order sounds very natural.

What matters most is:

• が marks お腹
• で marks コンビニ
• を marks おにぎり

Those particles show the grammatical roles, so the sentence can tolerate some movement.

A casual version would usually be:

• お腹がすいたら、コンビニでおにぎりを買う。

The main change is:

• 買います → 買う

That changes the sentence from polite style to plain/casual style.

You might also hear very casual speech with omitted particles in conversation, but for standard casual Japanese, this is the clean version:

• お腹がすいたら、コンビニでおにぎりを買う。

Yes, literally it is related to the stomach becoming empty, but as a set expression it simply means to get hungry.

So:

• お腹がすく = literal image: the stomach becomes empty
• natural English meaning: to get hungry

Many Japanese expressions are best learned as a unit, and お腹がすく is one of them. Even though the literal idea is useful to know, you should remember the whole phrase as the normal way to say to be hungry / to get hungry.