tomodati ga kuru mae ni, surippa wo genkan ni narabete, thissyu mo tukue no ue ni oite okimasita.

In 友達が来る前に, 友達が来る is a smaller clause meaning the friend comes / the friend will come. Inside this kind of clause, the subject is usually marked with が.

Using は is not impossible, but it would sound more like you are specially contrasting or highlighting the friend as a topic. The neutral, standard form here is 友達が来る前に.

With 前に, Japanese normally uses the dictionary form of the verb:

• 来る前に = before coming / before someone comes

The idea is that, from the point of view of before, the action has not happened yet. So even if the whole sentence is in the past, Japanese still says 来る前に.

Compare:

• 来る前に = before coming
• 来た後で = after coming

So the tense in English may become before my friend came, but Japanese still uses 来る.

前 literally means front or before. In this sentence, it works like a noun meaning the time before.

So:

• 友達が来る前 = the time before my friend comes
• 友達が来る前に = before my friend comes

This is a very common pattern:

• Vる前に = before doing V
• Nの前に = before N / in front of N

並べて is the て-form of 並べる (to line up, to arrange).

Here, the て-form connects actions:

• スリッパを玄関に並べて、
• ティッシュも机の上に置いておきました。

So the meaning is basically:

• I lined up the slippers in the entryway, and also put tissues on the desk in advance.

The て-form often links actions in sequence, like and then or simply and.

Because the sentence has both:

• what is being arranged
• where it is being arranged

So:

• スリッパを = the slippers are the direct object
• 玄関に = the entrance area is the place where they are put/arranged

With verbs of placement like 置く, 並べる, 入れる, に often marks the destination or resulting location.

Not exactly. In Japanese, 玄関 usually means the entryway area of a home, often the place where people take off their shoes. It is closer to the idea of a genkan than just front door.

So スリッパを玄関に並べる suggests preparing slippers in the entrance area for a guest.

も means also or too.

It shows that putting out tissues is an additional action, on top of preparing the slippers.

So the flow is:

• I prepared the slippers,
• and also put tissues on the desk.

This is a very common noun-linking pattern:

• 机 = desk / table
• 上 = top / above
• 机の上 = on top of the desk / table
• 机の上に = onto / on the desk

The の connects the nouns, and に marks the place where the tissues were placed.

You will see the same pattern in expressions like:

• 箱の中 = inside the box
• 椅子の下 = under the chair
• 学校の前 = in front of the school

〜ておく is very important. It often means:

• do something in advance
• do something and leave it that way

So:

• 置きました = I put it there
• 置いておきました = I put it there beforehand / I placed it there and left it ready

In this sentence, 置いておきました gives the nuance of preparation for the friend's arrival.

The main verb is 置いておきました, which is past polite form, so the whole sentence describes something that was done in the past.

The 来る inside 友達が来る前に does not control the overall tense of the sentence in the same way English does. It is part of the fixed pattern Vる前に.

So the sentence means something like:

• Before my friend came, I had lined up slippers in the entryway and also put tissues on the desk.

Japanese often leaves out subjects when they are clear from context.

Here, the person who arranged the slippers and put out the tissues is naturally understood to be the speaker or narrator, so 私 does not need to be said.

This is very normal in Japanese. English usually needs I, but Japanese often does not.

Japanese nouns usually do not change form for singular and plural the way English nouns do.

So:

• 友達 can mean friend or friends
• ティッシュ can mean tissue or tissues

The exact number depends on context. English translations have to choose one, but the Japanese itself often leaves it unstated.

Yes. They are katakana words based on foreign vocabulary.

• スリッパ = slippers
• ティッシュ = tissue / tissues

But remember: even when a Japanese word comes from English, its usage may not be exactly the same as in English. For example, ティッシュ often strongly suggests facial tissues rather than just any kind of tissue.