haha ha toosuto ni bataa de ha naku mayoneezu wo nutte tabemasu.

は marks 母 as the topic: As for my mother, ...

So 母は sounds like the speaker is introducing what their mother does. This is very natural in a general statement.

If you used が instead, it would put more focus on mother as the one who does the action, often with a nuance like my mother is the one who... That is not impossible, but は is the more natural choice here.

With the verb ぬる (to spread / apply), the thing you put something onto is often marked with に.

So:

• トーストに = onto the toast
• マヨネーズを = the mayonnaise as the thing being spread

This is a very common pattern:

• パンにジャムをぬる = spread jam on bread
• 壁にペンキをぬる = apply paint to a wall

So here, に does not mean a destination like to school. It marks the surface or target of the spreading.

XではなくY means not X, but Y.

So:

• バターではなくマヨネーズ = not butter, but mayonnaise

This is a contrast pattern. The speaker is correcting or contrasting one item with another.

In this sentence, the contrast is between butter and mayonnaise.

Technically, it is the particle は, but in ではなく it is best understood as part of a set expression.

For learners, it is easiest to treat ではなく as one chunk meaning:

• not ... but ...

So you do not need to read it as a new topic marker for the sentence. It is part of the contrastive expression attached to バター.

Because マヨネーズ is the direct object of ぬる.

With ぬる, Japanese often works like this:

• surface / place → に
• thing being spread → を

So:

• トーストに = onto the toast
• マヨネーズをぬる = spread mayonnaise

That is why マヨネーズ takes を, not に.

ぬって is the て-form of ぬる.

The て-form is often used to connect verbs. Here it links two actions done by the same person:

1. spread mayonnaise on the toast
2. eat it

So ぬって食べます means something like:

• spreads it and eats it
• spreads it, then eats it

The final verb 食べます is the main verb of the sentence.

Yes, ぬって comes from a verb, but in this sentence it is not the final verb. It is a linking form.

Japanese sentences normally end with the main finite verb. Here that verb is 食べます.

So:

• ぬって = spread, and...
• 食べます = eats

Without 食べます, the sentence would only say that the mother spreads mayonnaise on toast. With 食べます, it says she spreads it and then eats it.

The object is understood from context: it is the toast.

Japanese often leaves out words that are obvious. After hearing トーストにマヨネーズをぬって, it is natural to understand that the thing being eaten is the toast.

A fully explicit version could say the toast again, but that would sound repetitive:

• 母はトーストにバターではなくマヨネーズをぬって、トーストを食べます。

That is grammatical, but the original sentence is more natural because Japanese often omits repeated information.

Yes. You could say:

• 母はトーストにバターじゃなくてマヨネーズをぬって食べます。

That sounds very natural in casual spoken Japanese.

The difference is mostly style:

• ではなく = a bit more formal or written
• じゃなくて = more conversational

The meaning is basically the same.

Not completely. Japanese word order is more flexible than English because the particles show each word’s role.

For example, this also works:

• 母はバターではなくマヨネーズをトーストにぬって食べます。

That said, the original order is very natural and easy to process:

• topic first: 母は
• place/surface: トーストに
• contrast: バターではなくマヨネーズを
• actions: ぬって食べます

So the sentence is well-formed as given.

母 is commonly used when talking to other people about your own mother.

お母さん is often used:

• when calling your own mother directly
• when talking about someone else’s mother
• in less formal family-style speech

So in a sentence like this, 母は is a very natural way to say my mother.

Normally, Japanese is written without spaces:

母はトーストにバターではなくマヨネーズをぬって食べます。

The spaces here are just for learners, to make the parts easier to see. They are not normally used in standard Japanese writing.