kaze ga tsuyoi hi wa, sentakumono ga hayaku kawaku.

It is a clause + noun pattern:

• 風が強い = the wind is strong
• 日 = day

So 風が強い日 literally means a day when the wind is strong.

A key point for English speakers: Japanese puts the describing clause before the noun. There is no word like when, that, or which here.

So the structure is:

• [風が強い] 日
• [the wind is strong] day

This is one of the most common Japanese patterns for making expressions like the person who came, the book I bought, the day when it rained, etc.

Here, 風 is the subject of the small clause 風が強い.

• 風が強い = the wind is strong

Using が is the normal, neutral choice inside this kind of noun-modifying clause.

If you used は instead, it would sound much more marked or contrastive, and in this sentence it would not be the natural way to say it.

So for a learner, the safest takeaway is:

• X が adjective/noun/verb + noun is a very common pattern
• 風が強い日 = a day when the wind is strong

Because 風が強い日 is the topic of the whole sentence.

So:

• 風が強い日は、洗濯物が早く乾く。

means something like:

• As for windy days, laundry dries quickly
• or more naturally, On windy days, laundry dries quickly

The は is not directly a word for on or when. It is the topic marker. But in English, topic phrases like this often get translated as on days when... or when...

Because there are really two clauses here, and each clause has its own subject.

You can break the sentence like this:

• [風が強い] 日は
• [洗濯物が早く乾く]

So:

1. 風が強い

   • subject: 風
   • predicate: 強い

2. 洗濯物が早く乾く

   • subject: 洗濯物
   • predicate: 乾く

This is completely normal in Japanese. A sentence can contain a smaller clause inside a bigger one.

Yes. 風の強い日 is also very natural and common.

Compare:

• 風が強い日 = a day when the wind is strong
• 風の強い日 = a windy day / a day with strong wind

The meaning is very close. In many situations, they are interchangeable.

A rough nuance difference:

• 風が強い日 feels a little more like a full clause: the day when the wind is strong
• 風の強い日 feels a little more compact and descriptive: a windy day

Both are good Japanese.

早く is the adverb form of the い-adjective 早い.

Rule:

• 早い → 早く

Here it modifies the verb 乾く:

• 早く乾く = dry quickly / dry sooner

So:

• 早い = adjective, used before nouns or as a predicate
• 早く = adverb, used to modify a verb

Examples:

• 早い電車 = an early train
• 電車が早く来る = the train comes early

In this sentence, 早く means the laundry dries more quickly / sooner.

This is a very common question.

Both 早い and 速い can be read はやい, but they mean slightly different things:

• 早い = early, soon
• 速い = fast in speed

In 早く乾く, the idea is usually that the laundry becomes dry sooner or in less time, so 早く is the usual choice.

If you think in English, it is closer to it dries sooner than to it moves fast.

Because 乾く is an intransitive verb, and 乾かす is a transitive verb.

• 乾く = to become dry / to dry
• 乾かす = to dry something

In this sentence, the laundry is the thing that dries:

• 洗濯物が乾く = the laundry dries

If you wanted to say that a person dries the laundry, you would use 乾かす:

• 洗濯物を乾かす = to dry the laundry

So the given sentence is focusing on what happens to the laundry naturally under windy conditions.

洗濯物 means laundry or clothes being washed / hung out to dry.

It is made from:

• 洗濯 = washing, laundry
• 物 = thing(s)

So literally it is something like washing things, but in natural English we usually just say laundry.

Depending on context, it can refer to:

• clothes that need washing
• clothes that have been washed
• clothes hanging outside to dry

In this sentence, it means the laundry that is drying.

It is read:

• せんたくもの

Not せんたくぶつ.

A few useful readings from the sentence:

• 風 = かぜ
• 強い = つよい
• 日 = ひ
• 洗濯物 = せんたくもの
• 早く = はやく
• 乾く = かわく

Because Japanese uses the plain non-past for general truths, habits, and regular tendencies, not just for the future.

So:

• 洗濯物が早く乾く here means laundry dries quickly
• not specifically will dry quickly one time in the future

The whole sentence expresses a general fact:

• On windy days, laundry dries quickly

If you wanted a polite version, you could say:

• 風が強い日は、洗濯物が早く乾きます。

The meaning would stay basically the same; only the politeness level changes.