tomodati ga kuru mae ni, haha ha karaage wo ookii sara ni moritukete okimasita.

Because 前 means before, and when you say before doing X, Japanese normally uses the dictionary form of the verb before 前.

• 友達が来る前に = before the friend comes
• 来た前に is not natural Japanese

A useful contrast:

• 来る前に = before coming / before someone comes
• 来た後で = after coming / after someone came

So with 前, use the plain non-past form:

• 食べる前に = before eating
• 寝る前に = before sleeping

Because 友達が来る is a clause modifying 前: the friend comes → before the friend comes.

Inside this kind of subordinate clause, が is very common for marking the subject.

• 友達が来る前に = before the friend comes

If you used 友達は, it would sound more contrastive or topical, as if you were comparing the friend with someone else. In a neutral sentence, が is the natural choice here.

Also notice that the whole sentence has two different roles:

• 友達が来る前に = time clause
• 母は = main sentence topic

So が marks the subject of the embedded clause, while は marks the topic of the main clause.

前に means before.

In this sentence:

• 友達が来る前に = before the friend comes

The に marks the time point relative to the action in the main clause. It helps connect the idea of before X to what happens next.

You will often see this pattern:

• Verb dictionary form + 前に
• Noun + の + 前に

Examples:

• 出かける前に = before going out
• 寝る前に = before sleeping
• 食事の前に = before the meal

Because 母 is the topic of the main sentence.

The sentence is basically saying:

• As for Mother, she plated the karaage in advance before the friend came.

So:

• 母は = as for mother

This does not necessarily mean mother is the grammatical subject in the same way English uses subject; は mainly marks what the sentence is about.

In this sentence, it also helps separate the two parts clearly:

• 友達が来る前に = subordinate time clause
• 母は唐揚げを大きい皿に盛り付けておきました = main clause

〜ておく means to do something in advance or to do something and leave it that way for later convenience.

So:

• 盛り付けました = plated/served
• 盛り付けておきました = plated/served in advance

In this sentence, the nuance is important: the mother prepared the karaage ahead of time, before the friend arrived.

So 〜ておく often implies:

• preparation
• foresight
• getting something ready

Examples:

• 買っておきました = I bought it in advance
• 開けておきます = I’ll open it beforehand / I’ll leave it open

Because the past tense describes the main action: the mother already finished plating the karaage.

The sentence means that, from the speaker’s viewpoint, this preparation happened in the past:

• The friend’s coming is the reference point
• The mother’s plating happened before that
• And the whole situation is being reported as something completed

So:

• 来る in 友達が来る前に does not mean the coming is happening right now; it is just the plain form used before 前に
• 盛り付けておきました shows the main action was completed

This is normal in Japanese. The tense of the verb before 前に does not work the same way as English tense.

Because 唐揚げ is the direct object of 盛り付ける.

• 盛り付ける = to arrange/plate/serve food nicely
• What was plated? → 唐揚げ
• Therefore: 唐揚げを

So:

• 唐揚げを盛り付ける = to plate the karaage

Here, に marks the place/surface/container onto which the food is arranged.

• 大きい皿に盛り付ける = to plate it on/onto a big plate

So the plate is the destination or location of the arrangement.

This is similar to:

• 皿にのせる = put onto a plate
• テーブルに置く = place on the table

In this sentence:

• 唐揚げを = the thing being plated
• 大きい皿に = onto a big plate

Because 大きい is an i-adjective, and i-adjectives directly modify nouns without の.

• 大きい皿 = a big plate

This is just like:

• 小さい家 = a small house
• 新しい本 = a new book

You use の with nouns modifying nouns, or with some na-adjective-type structures, but not with normal i-adjectives like 大きい.

Yes, 大きな皿 is also possible.

Both mean a big plate, but there is a slight stylistic difference:

• 大きい皿 = straightforward adjective usage
• 大きな皿 = a slightly more descriptive/literary-feeling modifier

In everyday conversation, both are common and natural. For many learners, it is enough to know that both can modify a noun here.

The sentence follows a very typical Japanese structure:

• [time clause] + [topic] + [object] + [place] + [verb]

So:

• 友達が来る前に = before the friend comes
• 母は = mother, as for her
• 唐揚げを = karaage
• 大きい皿に = onto a big plate
• 盛り付けておきました = plated in advance

Japanese often puts background information first, and the main verb comes at the end.

By itself, 友達 can mean friend or friends, depending on context.

In this sentence, it is most naturally understood as a friend, because of the English meaning you were given, but Japanese does not always mark singular/plural clearly.

If the speaker wanted to make plurality clearer, they might use context or add something else, but often Japanese simply leaves it unspecified.

Those spaces are usually just for learners. Normal Japanese writing does not put spaces between words like this.

Normally, the sentence would be written as:

友達が来る前に、母は唐揚げを大きい皿に盛り付けておきました。

The spaces are there only to make the structure easier to read.