haha ha, kinoko to hourensou wo ireta atatakai udon wo yoru ni yoku tukurimasu.

は marks 母 as the topic: As for mother, ....
This sentence is talking about what the mother usually does, so は is very natural.

If you used が, it would sound more like you are specifically identifying the mother as the one who does it, often with a sense of contrast or new information.

• 母は = As for my mother, she often makes ...
• 母が = My mother is the one who makes ... / more emphasis on mother

Because there are two different things being marked as objects in two different parts of the sentence.

1. きのことほうれん草を入れた
   Here, を marks what was put in:
   put in mushrooms and spinach

2. うどんを作ります
   Here, を marks what is made:
   make udon

So the structure is:

• [きのことほうれん草を入れた] うどん = udon that has mushrooms and spinach in it
• そのうどんを作ります = makes that udon

入れた is the past/plain form of 入れる (to put in, add).

Here it directly modifies うどん, so:

• きのことほうれん草を入れたうどん
  = udon into which mushrooms and spinach were put
  = more naturally, udon with mushrooms and spinach added

In Japanese, verbs in plain form can modify nouns directly, kind of like a relative clause in English.

Great question. In Japanese, a past-form verb before a noun does not always mean the whole sentence is in the past.

Here, 入れた describes the state of the udon as udon that has had mushrooms and spinach added to it. It is part of the noun phrase, not the main verb of the sentence.

The main verb is 作ります, and that is what tells you the sentence is about a habitual action.

So:

• 入れた = added / containing
• 作ります = makes / often makes

Because 温かい is an い-adjective, and い-adjectives can directly modify nouns.

• 温かい うどん = warm udon

No な or の is needed.

Compare:

• 温かい うどん = correct
• きれいな うどん = if using a な-adjective, you need な
• 日本の うどん = if using a noun to modify another noun, you often use の

Both can relate to warmth/heat, but they are used differently.

• 温かい = warm, pleasantly warm, comforting
• 熱い = hot, physically hot, sometimes too hot to touch or eat comfortably

For food like soup or udon, 温かい often gives the sense of warm, comforting, served hot. It sounds natural here.

夜に means at night / in the evening.

The particle に marks a point in time. With time words, に is sometimes required, sometimes optional, depending on the word and style.

With 夜, both can be heard:

• 夜に = clearly marks the time, very standard
• 夜 = also possible in some contexts

In this sentence, 夜に sounds perfectly natural and clearly means at night.

よく means often here, and it modifies the main verb 作ります.

So:

• 夜によく作ります = often makes (it) at night

Japanese adverbs are often placed before the verb they modify.
You could think of the sentence as:

• Mother, as for warm udon with mushrooms and spinach added, often makes it at night.

In Japanese, the non-past form 作ります can cover:

• present/future
• habitual/repeated actions

Here, because of よく (often), it clearly means a habitual action:

• often makes

So this is not just will make one time, but usually makes.

と is the particle for and when listing nouns.

• きのことほうれん草 = mushrooms and spinach

This is a straightforward noun list.

A learner might also see や in other sentences:

• AとB = A and B
• AやB = A and B, among other things

Here, と suggests these are the ingredients being specifically mentioned together.

In a sentence like this, 母 usually means my mother when the speaker is talking about their own mother.

Japanese often omits words like my when the meaning is clear from context.

Also, 母 is the humble/plain way to refer to your own mother when speaking to others.
If you were talking directly to your mother or referring respectfully to someone else’s mother, you would often use お母さん instead.

A useful breakdown is:

• 母は = as for my mother
• きのことほうれん草を入れた = with mushrooms and spinach added
• 温かいうどんを = warm udon
• 夜に = at night
• よく = often
• 作ります = makes

So the grammar pattern is basically:

Topic + [noun-modifying clause] + noun + object marker + time + adverb + verb

This kind of long noun phrase is very common in Japanese:

• [きのことほうれん草を入れた温かいうどん]を作ります

That whole chunk is the thing being made.

The commas mainly help readability and pacing. They do not change the basic grammar.

Japanese punctuation is often more flexible than English punctuation. A writer may insert commas to make long phrases easier to read, especially in a sentence with several particles and modifiers.

So the commas here help separate:

• the topic: 母は
• the ingredients: きのことほうれん草を入れた
• the object: 温かいうどんを
• the time/adverb/verb: 夜によく作ります

But the sentence would still be grammatically understandable without them.