tenki o yosoushita.

direct object particle

Breakdown of tenki o yosoushita.

• 天気てんき: weather
• 予想よそう: an expectation; prediction

Questions & Answers about tenki o yosoushita.

What does を mean in this sentence?

を marks the direct object of the verb. Here, 天気 is the thing being predicted, so 天気を予想した means something like predicted the weather.

In very simple terms:

• 天気 = weather
• を = marks what the action is done to
• 予想した = predicted

Why is 天気 followed by を and not は?

Because 天気 is the object of 予想した, not the topic of the sentence.

• 天気を予想した = predicted the weather
• 天気は... would make weather the topic, and the sentence would usually continue in a different way

For example:

• 天気はどうなるかな。 = As for the weather, I wonder what will happen.

In your sentence, the speaker is talking about the action of predicting, so を is the natural choice.

What form is 予想した?