komugiko wo kau tuide ni, aonori mo katta.

小麦粉 = こむぎこ
を = を
買う = かう
ついでに = ついでに
青のり = あおのり
も = も
買った = かった

Full reading: こむぎこ を かう ついで に、あおのり も かった。

It means something like:

• while I was at it
• on the occasion of doing that
• since I was already doing X, I also did Y

So the sentence is not just listing two purchases. It specifically says that buying 青のり was an extra thing done because the speaker was already buying 小麦粉.

Because V-る + ついでに is a very common pattern when talking about an errand or planned action that creates an opportunity for another action.

Here, 小麦粉を買うついでに means roughly:

• while going to buy flour
• when buying flour
• since I was buying flour anyway

You can also say 買ったついでに, but that sounds more like:

• when I bought flour
• after buying flour

So both are possible, but 買うついでに is very natural when the first action is treated as the main errand.

This is a very common Japanese pattern. The main verb at the end, 買った, sets the overall sentence in the past.

The earlier clause, 小麦粉を買うついでに, is a modifying or subordinate phrase. In that kind of phrase, Japanese often uses the plain non-past form even when the whole situation is in the past.

So this does not mean the flour-buying happened in the future. The whole event is past; 買う is just the normal form used in this construction.

Yes. ついで is basically a noun meaning:

• opportunity
• occasion
• a convenient extra stop / extra action

That is why it can take に:

• 買うついでに = on the occasion of buying
• 行くついでに = while going / when going anyway

So the structure is:

verb + ついでに

The に marks the occasion or situation in which the main action happens.

So:

• 小麦粉を買うついでに = on the occasion of buying flour
• then 青のりも買った = I also bought aonori

In other words, に helps connect the occasion phrase to the main action.

Because も means also / too.

When も is attached to a direct object, it usually replaces を:

• 青のりを買った = I bought aonori
• 青のりも買った = I bought aonori too

So も is showing that 青のり was an additional purchase.
A form like 青のりをも買った is possible, but it sounds more emphatic or formal than the normal everyday phrasing.

Usually, yes.

Xを買うついでにYも買った strongly suggests:

• the speaker bought X
• and, while doing that, also bought Y

The sentence focuses on the extra purchase, 青のり, but it normally implies that the flour was bought as well.

You can say that, but the nuance is different.

• 小麦粉を買って、青のりも買った = I bought flour, and I also bought aonori
• 小麦粉を買うついでに、青のりも買った = While I was at it buying flour, I also bought aonori

The ついでに version adds the idea that the second action was a convenient extra, not just another event in sequence.

Japanese often leaves out the subject when it is obvious from context.

In this sentence, the same person is understood for both verbs:

• 小麦粉を買う = (I) buy flour
• 青のりも買った = (I) also bought aonori

In English, we usually supply I, but Japanese does not need to say it if the context already makes it clear.