jagaimo no ryouri o tsukuru toki wa, zairyou o saki ni kitte oku to raku da.

Here, の connects two nouns:

• じゃがいも = potato / potatoes
• 料理 = dish, cooking, cuisine

So じゃがいもの料理 means a potato dish, potato cooking, or food made with potatoes.

This is a very common use of の: it shows that one noun describes or relates to another noun. It does not mean possession here in the English 's sense. It is more like potato-related dish or dish using potatoes.

Examples:

• 魚の料理 = fish dish
• 野菜のスープ = vegetable soup

とき means when. The phrase 作るとき means when (you) make it or at the time of making it.

Adding は after that marks the whole when phrase as the topic:

• 作るとき = when making it
• 作るときは = as for when making it / when you make it

This は gives a sense like:

• when it comes to making potato dishes...
• when you make potato dishes...

It often introduces advice, a general rule, or a contrast.

Yes, and that is exactly why を is used.

作る means to make, and the thing being made is the direct object, so it takes を:

• 料理を作る = to make a dish / to cook

This is just standard object marking.

Examples:

• ご飯を作る = make a meal
• ケーキを作る = make a cake

先に means first, ahead of time, or beforehand.

In this sentence, it means the ingredients should be cut before the main cooking process continues.

So:

• 材料を先に切っておく = cut the ingredients first / cut the ingredients in advance

It often suggests doing something early because it will help later.

Examples:

• 先に食べてください。 = Please eat first.
• 先に準備しておく。 = Prepare it in advance.

This is a very important grammar pattern.

• 切る = to cut
• 切っておく = to cut and leave it that way for later / to cut in advance

The pattern ておく means:

1. do something in preparation for later, or
2. do something and leave it as it is

So here, 材料を切っておく means not just cut the ingredients, but cut them beforehand so they are ready when needed.

Compare:

• 材料を切る = cut the ingredients
• 材料を切っておく = cut the ingredients ahead of time

This と means if / when, and it connects a condition to a result.

• 材料を先に切っておくと楽だ = If you cut the ingredients in advance, it is easier.

This と is often used for natural results, general truths, habits, or things that usually happen.

Examples:

• 早く寝ると元気になる。 = If you go to bed early, you feel better.
• このボタンを押すと、ドアが開く。 = If you press this button, the door opens.

In this sentence, it gives general advice: doing this leads to an easier process.

楽だ means easy, comfortable, less troublesome, or less effortful. It often suggests that something is more convenient or less of a hassle.

So 楽だ here means something like:

• it makes things easier
• it is less work
• it is more convenient

This is a little different from 簡単だ, which focuses more on something being simple or not difficult in a technical sense.

Compare:

• 簡単だ = simple, easy to understand/do
• 楽だ = comfortable, convenient, low-effort, less tiring

In cooking advice, 楽だ sounds very natural because it emphasizes reducing effort.

Japanese often leaves the subject unstated when it is clear from context.

In this sentence, the implied subject is something like:

• you
• one
• people in general

So the sentence gives general advice: When making potato dishes, cutting the ingredients beforehand makes things easier.

This omission is extremely common in Japanese.

In everyday Japanese, 材料 can mean both materials and ingredients, depending on context.

• In cooking, 材料 usually means ingredients
• In crafts or construction, it can mean materials

Since this sentence is about cooking, 材料 clearly means ingredients.

Examples:

• カレーの材料 = ingredients for curry
• 木の材料 = wooden materials

Yes. Japanese nouns usually do not show singular vs. plural clearly, so 料理 can mean:

• a dish
• dishes
• cooking
• cuisine

And じゃがいもの料理 can refer to:

• a potato dish
• potato dishes
• cooking with potatoes

The exact nuance depends on context. In this sentence, it sounds general, like when making potato dishes.

Yes. じゃがいもの料理を作るときは sets the situation or context for the main statement.

Structure:

• じゃがいもの料理を作るときは = when making potato dishes...
• 材料を先に切っておくと楽だ = cutting the ingredients in advance makes it easier

So the first part tells you when the advice applies, and the second part gives the advice/result.

Yes. The plain style ending 楽だ could be made polite as 楽です.

A polite version would be:

• じゃがいもの料理を作るときは、材料を先に切っておくと楽です。

That is a normal polite spoken or written version.

You could also make the whole sentence more formal depending on context, but 楽です is the simplest polite change.

No, it is not grammatically required, but it is helpful.

The comma after は separates the topic-setting phrase from the main statement:

• じゃがいもの料理を作るときは、材料を先に切っておくと楽だ。

Without the comma, the sentence is still understandable. The comma just makes it easier to read by showing the pause after when making potato dishes.