sizukana kafe de tomodati to issyo ni nihongo wo rensyuusimasu.

Because 静か is a na-adjective in Japanese, not an i-adjective.

• i-adjective: you attach it directly to a noun
  
  • 大きい犬 = a big dog
• na-adjective: you add な before a noun
  
  • 静かなカフェ = a quiet café

So:

• 静か = quiet
• 静かなカフェ = a quiet café

You do not say 静かい.

で marks the place where an action happens.

In this sentence, 練習します is an action, so カフェで means at/in the café as the location where the practicing takes place.

Compare:

• カフェで練習します = practice at a café
• カフェに行きます = go to a café

A useful contrast:

• に often marks destination or existence
• で often marks location of an action

So here, because the person is doing something in the café, で is correct.

Both help express the idea of doing something with a friend / together with a friend.

• 友達と = with a friend
• 一緒に = together

So 友達と一緒に is a very natural way to say together with a friend.

It may feel slightly repetitive in English, but in Japanese it is common and natural. It adds emphasis to the sense of doing the activity together.

Yes, but the nuance changes a little.

• 友達と日本語を練習します。
  = I practice Japanese with a friend.

• 友達と一緒に日本語を練習します。
  = I practice Japanese together with a friend.
  This feels a bit more explicit about doing it jointly.

• 一緒に日本語を練習します。
  = I practice Japanese together.
  This works only if the other person is already understood from context.

So in your sentence, using both is natural and clear.

Because 一緒に is the normal adverbial form meaning together.

Here, 一緒に describes how the action is done:

• 一緒に練習します = practice together

You can think of it as a set expression. In this sentence, 一緒に functions like an adverb.

を marks the direct object of the verb.

Here, the thing being practiced is 日本語, so:

• 日本語を練習します = practice Japanese

That is why 日本語 takes を.

Both are possible, but they are structured differently.

1. 日本語を練習します

This directly treats 日本語 as what you practice. It is very common and natural.

2. 日本語の練習をします

This literally means do Japanese practice or do practice of Japanese.

Both are correct. The first is usually simpler and more direct in everyday speech.

You may also hear:

• 日本語を勉強します = study Japanese
• 日本語を練習します = practice Japanese

The second often suggests practicing speaking, writing, pronunciation, etc., rather than studying in a broad sense.

練習します is the polite non-past form.

That means it can refer to:

• a present habit: I practice
• a future action: I will practice

Japanese non-past often covers both present and future depending on context.

Also, 練習する is a suru-verb:

• dictionary form: 練習する
• polite form: 練習します

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

From context, it could mean:

• I practice Japanese...
• We practice Japanese...
• someone else, if already understood

Japanese often leaves out subjects when they are obvious from the situation. So even though English usually needs a subject, Japanese often does not.

Japanese word order is more flexible than English because particles show each word’s role.

This sentence follows a very common pattern:

• description + place
• companion
• object
• verb at the end

So:

• 静かなカフェで
• 友達と一緒に
• 日本語を
• 練習します

The verb usually comes at the end, but other parts can often move around if the particles stay attached.

For example, these are also possible:

• 友達と一緒に静かなカフェで日本語を練習します。
• 日本語を静かなカフェで友達と一緒に練習します。

They all mean roughly the same thing, though the emphasis can shift slightly.

Yes, カフェ is very natural in modern Japanese.

It usually refers to a café in the modern sense. Another word you might see is:

• 喫茶店 = coffee shop / tea shop / café

The nuance can differ a little:

• カフェ often sounds more modern or stylish
• 喫茶店 can sound more traditional

So 静かなカフェ is a very natural phrase.

Those spaces are usually just for learner readability.

In normal Japanese writing, the sentence would usually be written without spaces:

静かなカフェで友達と一緒に日本語を練習します。

Japanese normally uses particles, kana, and kanji to show structure, so spaces are not generally needed.