مهما كانت القاعدة غير مستوية، يشكل صديقي الطين.

Here مهما introduces a concessive idea: no matter how / مهما يكن الأمر. It sets up the first clause as something that does not prevent the action in the second clause.

So the structure is roughly:

• مهما كانت القاعدة غير مستوية = No matter how uneven the base is
• يشكل صديقي الطين = my friend shapes the clay

A learner should notice that مهما often introduces a clause meaning regardless of... or no matter....

Because القاعدة is a feminine singular noun, and the verb كان must agree with it in gender.

• masculine: كان
• feminine: كانت

Since القاعدة is feminine, Arabic uses كانت.

In Arabic, قاعدة is a feminine noun. One very common sign of feminine nouns is the ـة ending, called tāʾ marbūṭa.

So:

• قاعدة = feminine
• therefore: كانت القاعدة and not كان القاعدة

Even when a noun refers to an inanimate thing, it still has grammatical gender in Arabic, and verbs/adjectives must agree with that gender.

Both are possible, but they work a little differently.

• القاعدة غير مستوية = the base is uneven / not level
• القاعدة ليست مستوية = the base is not level

Using غير + adjective is a very common way in Arabic to express non-... / not ... in a compact descriptive way.

So غير مستوية functions like uneven or not level, and it sounds quite natural in formal Arabic.

Because adjectives agree with the nouns they describe.

Here the noun is:

• القاعدة = feminine singular

So the adjective must also be feminine singular:

• مستوية

That is why you get:

• القاعدة غير مستوية

not:

• القاعدة غير مستوٍ
• القاعدة غير مستوٍة with the wrong form

غير means non- / not and is often followed by an adjective or noun.

In this sentence:

• غير مستوية = not level / uneven

Grammatically, غير behaves like a noun in an iḍāfa-like relationship with the word after it, so the following adjective is typically in the genitive when fully vocalized:

• غيرَ مستويةٍ

You do not need to think about that every time in normal reading, but it helps explain the formal grammar.

Arabic often prefers verb-first word order, especially in formal written style.

So:

• يشكل صديقي الطين = verb + subject + object
• صديقي يشكل الطين = subject + verb + object

Both are possible. The version in your sentence is perfectly normal MSA word order.

A useful thing to remember:

• VSO is very common in formal Arabic.
• SVO is also possible and can sometimes feel slightly more emphatic on the subject.

صديقي means my friend.

It is made from:

• صديق = friend
• ـي = my

So this is a noun with a attached possessive suffix.

In the sentence, صديقي is the subject of يشكل.

Because of the ـي suffix, the word does not look like a regular fully case-marked noun in basic spelling. Learners should simply recognize it as:

• صديقي = my friend

الطين is the direct object of يشكل.

So the structure is:

• يشكل = shapes
• صديقي = my friend
• الطين = the clay

In other words:

• My friend shapes the clay

In fully vocalized formal Arabic, it would be accusative:

• الطينَ

So a fully marked version would be: يُشَكِّلُ صديقي الطينَ

Here يشكل means shapes / forms / molds.

With الطين (clay), this verb is very natural, because it suggests giving clay a shape with the hands or as part of making something.

Depending on context, English could translate it as:

• shapes
• molds
• forms

So yes, يشكل الطين is a sensible combination.

Yes. In Arabic, after words like مهما, a past form can be used to express a general or timeless condition, not just a literal past event.

So:

• مهما كانت القاعدة غير مستوية does not necessarily mean only no matter how uneven the base was
• it can mean no matter how uneven the base is / may be

Meanwhile, يشكل is an imperfect verb and can express a present or general action:

• my friend shapes the clay

So the sentence is not really mixing tenses in a strange way; it is using normal Arabic style.

A fully vocalized version would be approximately:

مَهْمَا كَانَتِ الْقَاعِدَةُ غَيْرَ مُسْتَوِيَةٍ، يُشَكِّلُ صَدِيقِي الطِّينَ.

Key endings:

• القاعدةُ = nominative, because it is the subject of كانت
• غيرَ = accusative, as the predicate of كانت
• مستويةٍ = genitive after غير
• الطينَ = accusative, because it is the direct object of يشكل

In everyday unvocalized Arabic, these endings are usually not written.

It is understandable and grammatically workable, but a learner might also hear slightly different phrasing depending on context.

For example, a native speaker might also say:

• حتى لو كانت القاعدة غير مستوية، يشكل صديقي الطين.
• مهما تكن القاعدة غير مستوية، يشكل صديقي الطين.

These alternatives can sound a bit more explicitly like even if or no matter how.

So the given sentence is useful for learning, especially for grammar, but in real usage there may be stylistic variations.