أمي تطبخ وجبة كبيرة في المساء، وأنا معها في المطبخ.

Because Arabic normally leaves out short vowels in everyday writing. The word أمي is the noun أم meaning mother plus the suffix ـي meaning my.

So:

• أم = umm = mother
• أمي = ummī = my mother

The أ at the beginning is just the seat for the initial hamza; it does not automatically mean the vowel is a. In this word, the actual vowel is u, but that short vowel is not written in normal text.

In the present tense, Arabic verbs change form depending on the subject. The تـ here shows the verb is conjugated for she (or sometimes you masculine singular, depending on context).

So:

• تطبخ = she cooks / she is cooking
• In this sentence, because the subject is أمي (my mother), it clearly means she cooks / she is cooking

This is the present-tense form of the verb طبخ = to cook.

It can mean either, depending on context. In Modern Standard Arabic, the simple present tense often covers both:

• she cooks
• she is cooking

So أمي تطبخ could mean either my mother cooks or my mother is cooking. The surrounding context tells you which is more natural.

In Arabic, adjectives normally come after the noun they describe.

So the order is:

• وجبة = meal
• كبيرة = big

Together:

• وجبة كبيرة = a big meal

Also, the adjective must agree with the noun in:

• gender
• number
• definiteness
• case

Here both words are:

• feminine singular
• indefinite

That is why you get وجبة كبيرة.

Because وجبة is a feminine noun, and Arabic adjectives must agree with the nouns they describe.

So:

• وجبة is feminine
• therefore the adjective must also be feminine
• كبير = big masculine
• كبيرة = big feminine

That is why Arabic uses وجبة كبيرة.

Arabic often uses the definite article in common time expressions such as:

• في الصباح = in the morning
• في المساء = in the evening
• في الليل = at night

So في المساء is the normal way to say in the evening. It does not necessarily mean one very specific evening; it is just the usual Arabic expression.

معها is made of:

• مع = with
• ها = her

Together:

• معها = with her

In Arabic, object pronouns are often attached directly to prepositions. So instead of using a separate word for her, Arabic usually adds the pronoun to the preposition itself.

Examples:

• معي = with me
• معك = with you
• معها = with her
• معهم = with them

So معها is completely normal and very common.

Because Arabic usually does not use a present-tense form of to be in sentences like this.

In English, you say:

• I am with her in the kitchen

In Arabic, the present tense simply says:

• أنا معها في المطبخ
• literally: I with her in the kitchen

This is called a nominal sentence. The idea of am is understood automatically.

Yes. In Arabic, that is a perfectly complete sentence.

It has:

• وأنا = and I
• معها = with her
• في المطبخ = in the kitchen

Arabic often forms present-tense statements without an explicit verb to be. So this clause is fully grammatical and natural in MSA.

المطبخ means the kitchen, and in this context Arabic naturally uses the definite form because it refers to a known, specific kitchen — the one in the house or the relevant setting.

So:

• في المطبخ = in the kitchen
• في مطبخ = in a kitchen

The indefinite version would sound less natural here unless you specifically meant some kitchen rather than a particular one.

A fully vocalized version would be:

أُمِّي تَطْبُخُ وَجْبَةً كَبِيرَةً فِي المَسَاءِ، وَأَنَا مَعَهَا فِي المَطْبَخِ.

A few things to notice:

• تَطْبُخُ ends in ـُ because it is a present-tense indicative verb
• وَجْبَةً كَبِيرَةً are in the accusative because they are the direct object and its adjective
• فِي المَسَاءِ and فِي المَطْبَخِ are in the genitive after في

In normal Arabic writing, these short vowels are usually omitted.

Here the ل in الـ is pronounced clearly, because م is a moon letter, not a sun letter.

So you say:

• المساء = al-masāʾ
• المطبخ = al-maṭbakh

If the next letter were a sun letter, the l sound would assimilate, but that does not happen here.

The given order is natural, but Arabic word order is fairly flexible.

For example, you could also say:

• في المساء، أمي تطبخ وجبة كبيرة.

This puts more emphasis on in the evening.

You could also move parts of the second clause around, though some orders sound more natural than others. The original sentence is a very straightforward, learner-friendly way to say it.