te ga kayui toki ha, kono kuriimu wo sukosi tukeru.

Because かゆい is an i-adjective meaning itchy, and the body part that has that condition is often marked with が in Japanese.

So:

• 手がかゆい = My hand is itchy / The hand itches
• Literally, it is closer to As for the hand, it is itchy than to I itch the hand

Using を would not work here, because かゆい is not an action verb acting on hand. It describes a state or condition.

This pattern is very common with body parts and sensations:

• 頭が痛い = My head hurts
• のどが痛い = My throat hurts
• 目がかゆい = My eyes are itchy

とき means when or at the time when.

So:

• 手がかゆいとき = when your hand is itchy
• 手がかゆいときは adds は, which marks that whole time expression as the topic: as for when your hand is itchy...

This gives the sentence a conditional/situational feel:

• 手がかゆいときは、このクリームを少しつける。 = When your hand is itchy, apply a little of this cream.

The は often adds a sense like:

• in that case
• in that situation
• as for those times

The は marks 手がかゆいとき as the topic or situation being discussed.

Without は:

• 手がかゆいとき、このクリームを少しつける。

This is still natural and means basically the same thing: When your hand is itchy, apply a little of this cream.

With は:

• 手がかゆいときは...

it sounds a little more like:

• As for when your hand is itchy...
• In cases when your hand is itchy...

So は is not absolutely required, but it helps frame the sentence as advice or a general rule.

この means this, and it modifies a noun directly.

• このクリーム = this cream

It is used when the speaker is referring to a specific cream, probably one they are holding, showing, or have already mentioned.

Compare:

• このクリーム = this cream
• そのクリーム = that cream (near the listener or previously mentioned)
• あのクリーム = that cream over there

If you said just クリーム, it would mean simply cream in a more general way.

少し means a little or a small amount.

In this sentence:

• このクリームを少しつける = apply a little of this cream

It tells you the amount to apply.

Japanese adverbs like 少し are quite flexible in position, but this placement is very common:

• このクリームを少しつける
• 少しこのクリームをつける (possible, but less neutral here)

The most natural interpretation is that you apply a small amount of the cream, not that you do the action only a little bit in some abstract sense.

Yes, つける has several meanings in Japanese, and here it means to apply or to put on something onto a surface.

Common meanings of つける include:

• to attach / put on
• to turn on
  
  • 電気をつける = turn on the light
• to wear some things
  
  • ネクタイをつける = wear a tie
• to apply
  
  • 薬をつける = apply medicine
  • クリームをつける = apply cream

So in this sentence, つける means to put the cream onto your hand/skin.

Yes. 塗る is also very natural for cream, ointment, lotion, medicine, paint, etc.

Compare:

• クリームをつける = put/apply cream on
• クリームを塗る = spread/apply cream on

塗る emphasizes the act of spreading something over a surface. つける is broader and can sound slightly more general: put on / apply.

So:

• 手がかゆいときは、このクリームを少し塗る。

would also be natural.

In many real situations, both are fine.

The dictionary form here gives a general instruction, habit, or plain-style advice.

So this sentence can sound like:

• a note in a manual
• a simple instruction
• a neutral statement of what one does in that situation

Possible interpretations:

• When your hand is itchy, apply a little of this cream.
• When your hands get itchy, you use a little of this cream.

If you wanted a more direct polite instruction, you might say:

• 手がかゆいときは、このクリームを少しつけてください。 = When your hand is itchy, please apply a little of this cream.

So the plain form is not wrong at all; it is just less explicitly polite.

Yes, in a way. Japanese often leaves out the subject when it is obvious from context.

The sentence does not explicitly say:

• you
• I
• he/she

So depending on context, it could mean:

• When your hand is itchy, apply a little of this cream.
• When my hand is itchy, I apply a little of this cream.
• When the hand gets itchy, apply a little of this cream.

In everyday Japanese, this kind of omission is completely normal. Context tells you who the sentence is about.

It can mean either, depending on context.

Japanese often does not mark singular/plural the way English does. So:

• 手がかゆい can mean my hand is itchy
• or my hands are itchy

English usually forces you to choose hand or hands, but Japanese often leaves that open unless the exact number matters.

If the speaker wanted to be more explicit, they could say:

• 手がかゆい = hand(s) itchy
• 両手がかゆい = both hands are itchy

Japanese sentence order is different from English. A very common pattern is:

• time/situation
• object
• adverb
• verb

So here:

• 手がかゆいときは = when your hand is itchy
• このクリームを = this cream
• 少し = a little
• つける = apply

Japanese verbs usually come at the end of the clause or sentence. That is why つける comes last.

A very literal breakdown would be:

• When the hand is itchy, this cream a little apply.

Of course, that sounds unnatural in English, but it matches the Japanese structure more closely.

No, the comma is not strictly necessary, but it is helpful.

• 手がかゆいときは、このクリームを少しつける。
• 手がかゆいときはこのクリームを少しつける。

Both are fine.

The comma makes the sentence easier to read by separating the time/condition part from the main action:

• 手がかゆいときは | このクリームを少しつける

In Japanese writing, commas are often used for readability and rhythm, not only by strict grammatical rules.

No. Normal Japanese is usually written without spaces.

So the standard writing would be:

• 手がかゆいときは、このクリームを少しつける。

Spaces are often added in beginner materials to make it easier to see the words and particles:

• 手　が　かゆい　とき　は、　この　クリーム　を　少し　つける。

That is just a learning aid, not standard Japanese formatting.