مهما كانت فقاعة الهواء صغيرة، يعجن صديقي الطين.

Here مهما means something like no matter how or however.

So:

• مهما كانت فقاعة الهواء صغيرة = no matter how small the air bubble is

It introduces a concessive idea: the action in the main clause happens anyway, regardless of that condition.

In Arabic, especially with words like مهما, a past-tense form can be used to express a general condition or concession, not just past time.

So كانت here does not necessarily mean literal past time. In English, the most natural translation is often present:

• مهما كانت ... صغيرة = no matter how small ... is

This is a very normal Arabic pattern.

Because the subject of كانت is فقاعة الهواء, and the head noun there is فقاعة, which is feminine singular.

• فقاعة = bubble
• feminine singular noun
• so the verb is كانت rather than كان

The gender agreement follows فقاعة, not الهواء.

This is an iḍāfa construction, often called a construct phrase.

• فقاعة = bubble
• الهواء = the air

Together:

• فقاعة الهواء = the air bubble / bubble of air

A few important things about iḍāfa:

• the first noun usually does not take ال
• the second noun can be definite
• the whole phrase becomes definite because the second noun is definite

So:

• فقاعة الهواء = the air bubble
• if you wanted an air bubble, you would more likely say فقاعةُ هواءٍ

Because صغيرة here is not an attributive adjective inside the noun phrase. It is the predicate of كانت.

Compare:

• فقاعةُ الهواءِ الصغيرةُ = the small air bubble
  
  • here الصغيرة is an adjective describing the noun inside the same phrase

But in your sentence:

• كانت فقاعةُ الهواءِ صغيرةً = the air bubble was small / is small

Here صغيرة is a predicate, so it is normally indefinite, not definite.

That is why صغيرة is correct, not الصغيرة.

Because it refers to فقاعة, which is feminine singular.

In Arabic, adjectives and predicate adjectives usually agree with the noun in:

• gender
• number

So:

• فقاعة = feminine singular
• صغيرة = feminine singular

That agreement is completely normal.

In normal Arabic writing, short case endings are usually not written, but grammatically they are still there.

Relevant endings here would be:

• فقاعةُ — nominative, because it is the subject of كانت
• الهواءِ — genitive, because it is the second term of the iḍāfa
• صغيرةً — accusative, because it is the predicate of كان/كانت
• الطينَ — accusative, because it is the direct object of يعجن

So even though the sentence is usually written without these endings, a learner should know they are part of the grammar.

صديقي means my friend.

It is made of:

• صديق = friend
• ـي = my

So Arabic often expresses possession by attaching a suffix directly to the noun.

Examples:

• صديقي = my friend
• كتابي = my book
• بيتي = my house

So there is no need for a separate word meaning my here.

الطين literally means the clay.

Arabic often uses ال with materials, substances, or generic nouns more often than English does. Depending on context, الطين may be translated as:

• the clay
• clay

So the Arabic can sound natural even where English might or might not use the.

Because Arabic commonly allows, and often prefers, verb-first word order.

So:

• يعجن صديقي الطين = literally kneads my friend the clay
• natural English: my friend kneads the clay

This is a normal VSO order:

• Verb: يعجن
• Subject: صديقي
• Object: الطين

You could also say:

• صديقي يعجن الطين

That is also correct, but it gives the sentence a slightly different rhythm or emphasis.

At a more advanced grammar level, yes: مهما can function like a conditional particle, and the verb in the result clause may be analyzed as jussive.

But in normal unvocalized writing, you still just see:

• يعجن

because the final short vowel is not written.

So a beginner reading the sentence will not usually see any visible change in the spelling, even if a grammar book discusses the verb mood in more detail.