sumi wo katta.

It is read すみをかった (sumi o katta).

• 炭 = すみ (sumi) = charcoal
• を = object marker, pronounced お (o)
• 買った = かった (katta) = bought

を marks the direct object of the verb.

In this sentence, 炭 is the thing being bought, so it takes を:

• 炭を買った = [charcoal] [object marker] [bought]

With 買う (to buy), the thing you buy is normally marked by を.

In modern Japanese, を is usually pronounced o, even though it is historically associated with wo.

So:

• 炭を買った sounds like sumi o katta

You may still see it romanized as wo in some learning materials, but in normal speech it is basically o.

買う is the dictionary form, meaning to buy or buy.

買った is the plain past form, meaning bought.

So:

• 買う = buy / to buy
• 買った = bought

This sentence is in the past tense because the action has already happened.

This is a normal conjugation pattern for う-verbs.

買う is a u-verb, and verbs ending in -う often change like this in the plain past:

• 買う → 買った
• 使う → 使った
• 会う → 会った

So the う changes to った in the plain past.

The small っ shows a brief pause or doubled consonant sound.

So かった is pronounced katta, not kata.

That small pause matters. Compare:

• かた = kata
• かった = katta

In 買った, the small っ is part of the normal past-tense form.

The subject is omitted.

Japanese often leaves out information that is understood from context. So 炭を買った could mean:

• I bought charcoal
• He bought charcoal
• She bought charcoal
• They bought charcoal

The sentence itself does not say who did it. Context tells you.

The basic order is:

object + verb

So:

• 炭 = object
• を = marks the object
• 買った = verb

Japanese often puts the verb at the end of the sentence. That is one of the most common patterns learners notice.

It is plain/casual because the verb is 買った.

A polite version would be:

• 炭を買いました。

Both mean the same thing, but 買いました is more polite and is used more often in formal situations, with strangers, customers, teachers, and so on.

Japanese nouns usually do not show articles like a or the, and they usually do not change for singular vs. plural.

So 炭 can mean:

• charcoal
• some charcoal
• the charcoal

The exact interpretation depends on context.

That is why a short Japanese sentence can sound less specific than its English translation.

Not in the same role.

With 買う, the thing being bought is normally marked with を, so 炭を買った is the natural form.

You could add a topic or subject separately, for example:

• 私は炭を買った。 = I bought charcoal.
• 彼が炭を買った。 = He bought charcoal.

But 炭が買った would not work if you mean bought charcoal, because 炭 is the object, not the subject.

No. Japanese is normally written without spaces.

So the natural written form is:

• 炭を買った。

Spaces are often added in beginner materials to make the parts easier to see.