Learn Hardcore Japanese: yoru ni sakana wo tabemasu.

In Japanese, に marks a specific point in time when an action happens. Attaching に to 夜 turns it into “at night.” Without に, it might still be understood, but に makes it clear that the action occurs during that time.

Can we omit に after 夜?

Yes. With words like 朝, 昼, 夜 and days of the week, you can often drop に in casual speech and simply say: 夜、魚を食べます。
However, using に is more precise and typical in formal or written Japanese.

Why is 魚 followed by the particle を?

The particle を marks the direct object of a verb. Since 魚 (fish) is what is being eaten, you use 魚を to indicate “eat fish.”

Why is there no subject (like 私) in this sentence?

Japanese frequently omits the subject when it’s clear from context. Here it’s understood that “I” (私) eat fish at night. You only add 私は if you need to clarify or contrast, e.g. 私は夜に魚を食べます。

What are the readings of 夜 and 魚?

夜 is read よる (yoru, “night”) and 魚 is read さかな (sakana, “fish”).

Why does the verb 食べます come at the end?

Japanese uses Subject-Object-Verb (SOV) word order. You first state when the action happens (夜に), then what is affected by the action (魚を), and finally the action itself (食べます).

How would I make this sentence negative?

Change 食べます to its negative form 食べません: 夜に魚を食べません。
This means “I do not eat fish at night.”

How do I say “I ate fish at night” in the past tense?

Use the past polite form 食べました: 夜に魚を食べました。
That means “I ate fish at night.”

Can I use は instead of を after 魚?

If you say 魚は夜に食べます。, you’re making “fish” the topic: “As for fish, (I) eat it at night.”
It shifts the emphasis onto fish, perhaps implying you eat other foods at different times.