Learn Hardcore Japanese: watasi ha kinou tokei wo naosimasita.

The particle は marks 私 (“I”) as the topic of the sentence. It tells the listener what the sentence is about. Note that は is not always the grammatical subject marker—that’s が—but it sets up “I” as the context for what follows.

Why doesn’t 昨日 (yesterday) take a particle like に?

Time words such as 昨日, 今日, and 明日 often function as adverbial phrases on their own, so you can drop the particle. It’s perfectly natural to say 昨日時計を直しました. You could use 昨日に in certain formal contexts, but it’s uncommon.

Why does 時計 use the particle を?

The particle を marks the direct object of the verb. Here, 時計 (“clock/watch”) is what you fixed, so it takes を before the verb 直しました.

What is the dictionary form of 直しました, and what tense and politeness level is it?

The dictionary form is 直す (なおす).

直しました is the polite past tense (～ます form + した).
It turns “fix” into “fixed” in a polite register.

How would you express the same idea in casual (plain) speech?

In plain form you’d say:
私は昨日時計を直した。
Here, 直した is the plain past of 直す, and you keep は for the topic if you want to be explicit.

Can you omit 私 in this sentence?

Yes. Japanese often drops pronouns when context is clear. So 昨日時計を直しました still means “(I) fixed the clock yesterday,” especially if you’re already talking about what you did.

Why is the verb placed at the end, after the object and time expression?

Japanese follows a Subject-(Time)-(Object)-Verb order.

Time adverbs (昨日) → direct objects (時計を) → verb (直しました).
You can move the time phrase slightly (“時計を昨日直しました”), but the verb almost always comes last.

Could you substitute 修理しました for 直しました? Is there a nuance difference?

Yes, you could say 修理しました (しゅうりしました).

修理する often implies a more technical or mechanical repair.
直す is more general (“to fix,” “to correct”) and can apply to both physical objects and mistakes. Both are acceptable for a watch.

How do you write and pronounce 直す, and are there different kanji readings?

The verb is written 直す and pronounced なおす.
There is another verb 直す read ただす, but that one means “to correct” in writing or behavior. In everyday speech for “fixing,” you always use なおす.