kinou ha atama ga hidoku itakatta node, ie de yasumimasita.

は marks 昨日 as the topic: As for yesterday, ... or more naturally, Yesterday, ...

In this sentence, 昨日 is setting the time frame for the whole statement. Japanese often uses は for this kind of background information.

• 昨日は頭がひどく痛かったので、家で休みました。 = Yesterday, because my head hurt badly, I rested at home.

Using 昨日が would sound unnatural here, because が is not normally used just to set the time topic in this kind of sentence.

With expressions for physical symptoms in Japanese, the body part is often marked with が, not を.

So:

• 頭が痛い = My head hurts / I have a headache
• お腹が痛い = My stomach hurts
• 喉が痛い = My throat hurts

Even though English uses my head as something possessed by me, Japanese often treats the body part itself as the thing that has the property painful. So 頭が痛い literally feels more like the head is painful.

Japanese often leaves out things that are obvious from context. In a sentence about your own condition, 頭 naturally means my head.

So:

• 頭が痛い usually means My head hurts
• not The head hurts in some abstract sense

This kind of omission is very common in Japanese. Words like 私の are only added when needed for emphasis or clarity.

ひどく is the adverb form of ひどい, which means awful, terrible, or severe.

• ひどい = an -i adjective
• ひどく = adverb form, meaning badly, severely

So:

• 頭がひどく痛かった = my head hurt badly = I had a severe headache

This is the same pattern as other -i adjectives:

• 早い → 早く = fast → quickly
• 強い → 強く = strong → strongly
• 痛い does not become 痛く here because 痛かった is the main predicate, while ひどく modifies it.

痛かった is the past affirmative form of the -i adjective 痛い.

The pattern is:

• 痛い = hurts / painful
• 痛かった = hurt / was painful

For -i adjectives, the past form is made by changing -い to -かった:

• 高い → 高かった = was expensive / was tall
• 寒い → 寒かった = was cold
• 痛い → 痛かった = hurt / was painful

So 頭がひどく痛かった means my head hurt badly.

ので means because or so, and it gives a reason in a relatively neutral, explanatory, and often softer way.

• 頭がひどく痛かったので、家で休みました。 = Because my head hurt badly, I rested at home.

Compared with から:

• から can sound more direct or personal
• ので often sounds a bit softer and more matter-of-fact

For example:

• 頭が痛かったから、家で休みました。
• 頭が痛かったので、家で休みました。

Both are correct. In many contexts, ので sounds slightly more polite or formal.

で marks the place where an action happens.

• 家で休みました = rested at home

Here, 休みました is an action, so the location is marked with で.

By contrast, に is often used for:

• destination: 学校に行く = go to school
• existence: 家にいる = be at home
• specific time: 3時に = at 3 o’clock

So:

• 家で休む = rest at home
• 家にいる = be at home

Here, 休みました means rested or took it easy.

So:

• 家で休みました = I rested at home

Depending on context, 休む can also mean:

• to take a break
• to be absent from school or work
• to go to bed / rest

In this sentence, because it says 家で and gives a health reason, the meaning is clearly rested at home.

Japanese very often omits the subject when it is clear from context.

So instead of saying:

• 私は昨日、頭がひどく痛かったので、家で休みました。

Japanese often just says:

• 昨日は頭がひどく痛かったので、家で休みました。

The listener naturally understands that the speaker is talking about themselves unless context suggests otherwise.

This is one of the biggest differences from English: Japanese does not need to state I as often as English does.

They are marking different things.

• 昨日は: 昨日 is the topic
• 頭が: 頭 is the thing that has the condition painful

So the structure is roughly:

• As for yesterday, my head hurt badly, so I rested at home.

This is very normal in Japanese. A sentence can have a topic marked by は and, inside that sentence, another noun marked by が.

Yes. A common alternative is:

• 昨日は頭痛がひどかったので、家で休みました。

This means nearly the same thing:

• Yesterday, my headache was severe, so I rested at home.

The nuance is slightly different:

• 頭が痛い focuses on the head hurting
• 頭痛がひどい focuses on the headache as a condition

Both are natural.

The final verb is 休みました, which is polite past tense, so the sentence is in polite style overall.

Japanese sentences often mix plain forms before connectors like ので with a polite ending at the end of the sentence:

• 痛かったので、休みました。

This is completely normal.

So even though 痛かった is a plain form, the sentence still sounds polite because the main sentence ends politely with 休みました.

No, it is not required, but it is very natural.

Japanese often puts time expressions near the beginning of the sentence to set the scene:

• 昨日は頭がひどく痛かったので、家で休みました。

You could also say:

• 頭がひどく痛かったので、昨日は家で休みました。

But the original order sounds more natural if the main point is to describe what happened yesterday.

Japanese word order is flexible, but the beginning of the sentence is often used for topic and time information.