isoide kaku to, yoku matigai wo simasu.

Here, 急いで is the て-form of 急ぐ and it links to the next verb, 書く.

So 急いで書く means something like:

• to write while hurrying
• to write in a hurry

The て-form often connects actions or describes the manner in which something is done. In this sentence, it tells you how the writing is being done.

Both can relate to speed, but the nuance is different:

• 急いで書く = write in a hurry
• 速く書く = write quickly/fast

急いで suggests rushing, which fits well with the idea of making mistakes.
So the sentence implies not just speed, but careless speed caused by being in a rush.

Here, と is a conditional marker. It often means:

• when
• whenever
• sometimes if

So 急いで書くと、よく間違いをします means something like:

• When I write in a hurry, I often make mistakes
• If you write in a hurry, you often make mistakes
• Whenever someone writes in a hurry, mistakes often happen

With と, the result after it is usually something that naturally happens, regularly happens, or is seen as a general consequence.

Before conditional と, the verb normally appears in its plain form, like 書く.

This does not necessarily mean future tense here. In Japanese, plain non-past can also express:

• habits
• general truths
• repeated situations

So 書くと here means when/whenever one writes, not specifically will write.

よく means often here, and it modifies the whole action 間違いをします.

So:

• よく間違いをします = often make mistakes

Its position is natural because adverbs in Japanese often come before the verb or verb phrase they modify.

Be careful: よく can also mean well in other sentences, but here it clearly means often.

Both are possible, but they are built differently:

• 間違いをする = to make a mistake / to make mistakes
• 間違える = to make a mistake / to get something wrong

So this sentence uses the noun 間違い plus する:

• 間違い = mistake
• をする = do/make

Together: make mistakes

A very common alternative sentence would be:

• 急いで書くと、よく間違えます。

That is also natural.

を marks 間違い as the direct object of します.

So grammatically, Japanese is treating 間違い as the thing being done/made:

• 間違いをする = to make a mistake

This is similar to many Japanese noun + する expressions.

The subject is omitted, which is very normal in Japanese.

Depending on context, this sentence could mean:

• I often make mistakes when I write in a hurry
• You often make mistakes when you write in a hurry
• People often make mistakes when they write in a hurry

Japanese often leaves out the subject when it is understood from context.

It sounds like a general tendency or habitual situation, not a one-time event.

That comes from a few things working together:

• 書くと with conditional と
• よく meaning often
• the plain form 書く

So the overall sense is:

• Whenever writing is done in a hurry, mistakes often happen
• or more naturally, When I write in a hurry, I often make mistakes

Yes, it is understandable and natural.

However, many learners will also hear or see:

• 急いで書くと、よく間違えます。

That version uses 間違える instead of 間違いをする, but the meaning is very similar.

So the original sentence is fine; it is just one natural way to express the idea.

No, normal Japanese is usually written without spaces between words.

So the standard written form would be:

急いで書くと、よく間違いをします。

The spaces in your version are probably just there to help learners see the word boundaries more clearly.

Yes, either can work in English, depending on how natural you want the translation to sound.

Possible translations:

• If you write in a hurry, you often make mistakes.
• When I write in a hurry, I often make mistakes.
• Whenever you write in a hurry, you often make mistakes.

The most accurate feeling here is often when or whenever, because と tends to describe a result that regularly follows.