Learn Hardcore Japanese: kinou ha denti wo hutatu kaimasita.

は is the topic marker, so 昨日は means “As for yesterday….” It sets yesterday as the topic of the sentence and can add a nuance of contrast or emphasis (e.g. “Yesterday, at least, I bought batteries”).

Can’t we use に with 昨日 like in many other time expressions?

Yes. 昨日に電池を二つ買いました is grammatically correct. に simply indicates when something happened without making it the topic. Using は instead makes yesterday the topic and often implies “as for yesterday” (perhaps compared to other days).

What does the particle を do after 電池?

を marks the direct object of the verb 買いました, showing that 電池 (batteries) is what is being bought. Without を, the relationship between the noun and the verb would be unclear.

Why is 二つ read ふたつ and not に?

二つ uses the native Japanese counter つ, and when counting two items it’s pronounced ふたつ. The character 二 alone is read に only with Sino-Japanese counters like 個 (e.g. 二個 is にこ).

Could I say 電池を二個買いました instead of using 二つ?

Absolutely. 個 is a very common general counter for small, discrete objects. So 電池を二個買いました (にこ) sounds perfectly natural. 二つ is more generic and can count a wide range of things.

Why is the verb 買いました in this form rather than something like 買った?

買いました is the polite past tense of 買う. The ending ～ました makes it polite, and indicates a completed action (“bought”). In casual speech you’d typically use 買った.

There’s no subject in the sentence—how do we know who bought the batteries?

Japanese often omits the subject when it’s clear from context. Here, it’s implied that I (or we) bought the batteries. If you need to specify, you can add a pronoun with が or は, for example: 昨日は私が電池を二つ買いました.

Is the word order 固定, or can I rearrange the parts of the sentence?

Japanese word order is fairly flexible because particles mark each element. The most natural order is [time/topic] [object＋を] [number＋counter] [verb], i.e. 昨日は電池を二つ買いました. You could also say 昨日、電池を二つ買いました or even 電池を二つ昨日買いました, but generally the verb stays at the end.