مهما كانت البلاطة الخزفية كبيرة، يطليها صديقي.

Here مهما introduces a concessive idea: no matter how or however.

So:

• مهما كانت ... كبيرة = no matter how big ... is
• It does not mean simple whatever here; it is tied to the idea of size/degree because of كبيرة.

A very literal way to think of it is:

• مهما = whatever / no matter how
• كانت ... كبيرة = it was/is big

But in natural English, the whole clause is best understood as no matter how big the ceramic tile is.

Because it goes with البلاطة, and بلاطة is a feminine singular noun.

Arabic verbs agree with their subject in gender, so:

• masculine singular: كان
• feminine singular: كانت

Since the subject is البلاطة الخزفية = the ceramic tile, the sentence uses كانت.

This is a very common point for learners.

Even though كانت is formally the past form of كان, after مهما it often appears in a general concessive structure that does not have to refer to past time.

So in this sentence:

• مهما كانت البلاطة الخزفية كبيرة does not mean strictly no matter how big the tile was
• it means no matter how big the tile is / may be

In other words, Arabic often uses this form idiomatically in expressions like مهما كان/كانت... to give a general meaning rather than a real past-time meaning.

Yes, both are adjective-like words, but they do different jobs.

• الخزفية modifies البلاطة directly:
  البلاطة الخزفية = the ceramic tile

• كبيرة is the predicate of كانت:
  كانت البلاطة الخزفية كبيرة = the ceramic tile was/is big

So:

• الخزفية is an attributive adjective inside the noun phrase
• كبيرة is a predicate adjective describing the subject through كانت

That is why الخزفية comes right after البلاطة, while كبيرة comes later.

Because it is describing the definite noun البلاطة.

In Arabic, an attributive adjective normally matches the noun it describes in:

• definiteness
• gender
• number
• case

So:

• البلاطة = definite, feminine, singular
• الخزفية must also be definite, feminine, singular

That is why you get:

• البلاطة الخزفية = the ceramic tile

and not just البلاطة خزفية in this structure.

Because it describes البلاطة, which is feminine singular.

So the adjective must match the noun in gender and number:

• بلاطة → feminine singular
• كبيرة → feminine singular form of كبير

This is true even though كبيرة is the predicate of كانت, not a direct modifier like الخزفية.

كبيرة is the predicate of كان (called خبر كان), so it is in the accusative case.

With full vowel marking, it would be:

• كبيرةً

But in normal everyday Arabic writing, short vowels and tanwīn are usually not written, so you simply see:

• كبيرة

This can be confusing for learners, because the case is there grammatically even when it is not visible in the spelling.

يطليها can be split into:

• يطلي = he paints / coats / applies a coating to
• ها = it, feminine singular object pronoun

So:

• يطليها = he paints it or he coats it

The ها refers back to البلاطة, which is feminine singular.

Because Arabic often uses attached object pronouns instead of a separate word like English it.

Here the tile has already been mentioned:

• البلاطة الخزفية

So instead of repeating the noun, Arabic says:

• يطليها = he paints it

And because البلاطة is feminine singular, the pronoun must also be feminine singular:

• ها = her/it for a feminine singular noun

In this sentence, English translates it as it, not her.

Because Arabic very often uses a verb-first sentence order.

So this part is:

• يطليها صديقي
• literally: paints it my friend

Natural English reorders it as:

• my friend paints it

This is normal Arabic word order. You could also say:

• صديقي يطليها

but يطليها صديقي is perfectly natural and puts the action first.

صديقي means my friend.

It is made of:

• صديق = friend
• ـي = my

So:

• صديقي = my friend

In the sentence, صديقي is the subject of يطلي.

Because standard Arabic is usually written without short vowels.

So although the grammar includes case endings, normal spelling usually leaves them out.

A fully vocalized version would look something like:

• مهما كانتِ البلاطةُ الخزفيةُ كبيرةً، يطليها صديقي

This shows more clearly that:

• البلاطةُ is nominative
• الخزفيةُ matches it
• كبيرةً is accusative as the predicate of كانت

But in most real texts, those endings are not written, and readers understand them from grammar and context.