watasi ha yoru ni mizu wo nomimasen.

は is the topic marker. It tells the listener “I’m talking about me.” In this sentence, 私 (I) is the topic.

• が would mark the grammatical subject and is often used to introduce new information.
• Since 私 is known or given from context (“As for me…”), は is the correct choice.

The particle に marks a specific point in time when an action happens.

• 夜に means “at night.”
• Without に, 夜 can sound more like a general time-frame (often used with は, as in 夜は).

Yes, in casual speech learners sometimes omit に, but it changes the nuance:

• 夜に水を飲みません。 emphasizes the time “at night.”
• 夜水を飲みません。 is colloquial; it’s understood but less precise.
• If you use 夜は instead, you contrast “night” with other times: “As for at night, I don’t drink water.”

Japanese follows Subject-Time-Object-Verb (S-T-O-V) order:

1. 私（Subject）
2. 夜に（Time）
3. 水を（Object）
4. 飲みません（Verb）
   The verb always goes at the end, so 飲みません comes last.

The particle を marks the direct object of the verb.

• 水を飲む literally is “drink water.”
• 水を飲みません is “do not drink water.”

For ‐ます verbs (Type 1), drop ます and add ません:

1. Dictionary form: 飲む
2. Polite positive: 飲みます
3. Polite negative: 飲みません

Japanese often drops subjects when they’re clear from context.

• If it’s obvious you’re talking about yourself, you can simply say 夜に水を飲みません。
• You include 私 when you want to emphasize or clarify “I.”

Japanese present tense covers both habitual actions and near-future events.

• 夜に水を飲みません。 can mean:
  • Habitual: “I don’t drink water at night (ever).”
  • Future: “I’m not going to drink water tonight.”
  Context and additional time words (e.g., 今夜 for “tonight”) clarify which you mean.