عندما تفتح أمي الفرن، تضع فيه طبقا آخر من الطماطم واللحم.

عندما means when. It introduces a time clause:

• عندما تفتح أمي الفرن = when my mother opens the oven

It works a lot like English when, but in Arabic it is followed by a full clause. Here it sets up the situation before the main action happens.

They are in the present tense because the sentence describes a repeated or habitual action:

• تفتح = she opens
• تضع = she puts

So the sense is something like:

• Whenever my mother opens the oven, she puts another dish in it

In Arabic, the present tense is commonly used for routines and general truths like this.

In these verbs, تـ marks the 3rd person feminine singular in the present tense:

• تفتح = she opens
• تضع = she puts

That matches أمي (my mother), which is feminine.

A useful thing to know: this same تـ can also appear in forms meaning you. So by itself تفتح could mean she opens or you open. The subject أمي tells you that here it means she opens.

Because Arabic often uses verb–subject–object order, especially in formal written Arabic.

So:

• تفتح أمي الفرن = literally opens my mother the oven

This is normal in MSA. You could also say:

• أمي تفتح الفرن

That puts more emphasis on my mother as the subject. Both are possible, but the verb-first order is very common.

Arabic often leaves out subject pronouns when the subject is already clear.

In:

• تضع فيه طبقا آخر...

the verb form already tells you she puts, and the previous clause has already mentioned أمي. So Arabic does not need to repeat هي or أمي.

English usually repeats the subject:

• When my mother opens the oven, she puts...

Arabic often does not.

فيه means in it.

It is made of:

• في = in
• ـه = it / him

So:

• تضع فيه = she puts in it

Here ـه refers back to الفرن (the oven).

This is very common in Arabic: prepositions often attach directly to pronoun suffixes.

Arabic word order is flexible, and prepositional phrases often come before or after the direct object.

So this order:

• تضع فيه طبقا آخر

is perfectly natural and means:

• she puts another dish in it

Putting فيه earlier keeps the location closely tied to the verb. Arabic often does this more freely than English.

Because in Arabic, adjectives usually come after the noun they describe.

So:

• طبقًا = a dish
• آخرَ = another / other

Together:

• طبقًا آخرَ = another dish

Also, the adjective agrees with the noun in grammar. Here both are:

• masculine
• singular
• indefinite
• accusative

That is why the pattern is طبقًا آخرَ.

Here من means something like of or made with / consisting of.

So:

• طبق من الطماطم واللحم = a dish of tomatoes and meat

This is a very common use of من in Arabic when describing ingredients, contents, or what something is made from.

Compare:

• كوب من الماء = a cup of water
• طبق من الأرز = a dish of rice

Arabic often uses the definite article الـ with nouns where English uses a bare plural or a mass noun.

So:

• الطماطم = literally the tomatoes
• اللحم = literally the meat

But in natural English, this often translates simply as:

• tomatoes and meat

This is normal Arabic usage, especially with ingredients or categories of things.

Yes. In fully vocalized MSA, it would be:

عندما تفتحُ أمي الفرنَ، تضعُ فيه طبقًا آخرَ من الطماطمِ واللحمِ.

Some key endings are:

• تفتحُ: present tense, indicative
• الفرنَ: accusative, because it is the object of تفتح
• طبقًا: accusative, because it is the object of تضع
• آخرَ: accusative, agreeing with طبقًا
• الطماطمِ and اللحمِ: genitive, because they come after من

In normal Arabic writing, short vowels are usually omitted, so learners often do not see these endings written out.