تكييف الهواء جيد.

Because Arabic often uses nominal sentences in the present tense, and in those sentences there is usually no separate word for is / are / am.

So:

• تكييف الهواء جيد.
• literally: Air conditioning good.
• natural English: The air conditioning is good.

In past or future contexts, Arabic would use other forms, but in the simple present, leaving out is is completely normal.

Yes, it is a complete and natural sentence.

It is a nominal sentence made of:

• تكييف الهواء = the subject
• جيد = the predicate / description

So the structure is basically:

• [subject] + [description]

That is enough to make a full sentence in Arabic.

It is two words, and together they form a phrase:

• تكييف = conditioning / air-conditioning
• الهواء = the air

Together, تكييف الهواء literally means conditioning of the air, which is the Arabic way of saying air conditioning.

This is an iḍāfa construction, often called a genitive/possessive phrase. In this pattern:

• the first noun is followed by
• the second noun, which helps define it

So:

• تكييف الهواء = air conditioning
• literally: conditioning of the air

Because the noun being described is تكييف, and تكييف is masculine.

Even though the phrase is تكييف الهواء, the main noun in the phrase is still تكييف. The word الهواء is just the second part of the iḍāfa.

So the adjective/predicate agrees with تكييف, not with الهواء separately.

That is why we say:

• تكييف الهواء جيد

not:

• تكييف الهواء جيدة

Because here جيد is the predicate of the sentence, not an adjective inside the noun phrase.

Compare these two ideas:

• تكييف الهواء جيد = The air conditioning is good
• تكييف الهواء الجيد = the good air conditioning

In the sentence you gave, جيد is telling us something about تكييف الهواء, so it is normally indefinite.

That is why جيد appears without الـ.

A common pronunciation is:

takyīf al-hawāʾ jayyid

A rough guide:

• تكييف = tak-yeef
• الهواء = al-ha-waa’
• جيد = jay-yid

If full case endings are pronounced in careful MSA, it would be:

takyīfu l-hawāʾi jayyidun

But in everyday reading aloud, many speakers reduce or omit those final endings.

In fully vocalized MSA, the sentence would usually be:

تكييفُ الهواءِ جيدٌ

Here is why:

• تكييفُ has ḍamma because it is the subject of the nominal sentence
• الهواءِ has kasra because it is the second term of the iḍāfa
• جيدٌ has ḍamma + tanwīn because it is an indefinite predicate

So in very careful grammar, that is the fully marked form.

Yes, usually تكييف الهواء sounds more explicit and somewhat more formal.

You may also hear:

• التكييف جيد = The AC is good

This is shorter and very common in context.

But تكييف الهواء is perfectly good MSA and makes the meaning very clear.

Yes. In natural usage, تكييف الهواء can refer to:

• air conditioning in general
• the AC system
• the cooling setup in a room, building, or car

So depending on context, English might render it as:

• The air conditioning is good
• The AC is good
• The air-conditioning system is good

You can say:

تكييف الهواء جيد جدًا.

This adds جدًا, which means very.

So:

• جيد = good
• جيد جدًا = very good

That is a very good question, because Arabic often places adjectives after nouns.

The key difference is definiteness.

Here:

• تكييف الهواء is definite
• جيد is indefinite

So:

• تكييف الهواء جيد = The air conditioning is good
  because جيد is the predicate

But if you said:

• تكييف الهواء الجيد

then الجيد is definite too, so it becomes an adjective phrase:

• the good air conditioning

So matching definiteness helps you tell the difference.

Because in the phrase تكييف الهواء, the second noun is specific: it is the air.

In an iḍāfa construction, the definiteness of the whole phrase depends largely on the second noun. Since الهواء is definite, the whole phrase تكييف الهواء is understood as definite too.

That is why the sentence is interpreted as:

• The air conditioning is good

not just:

• Air conditioning is good

Not in this sentence.

Even though تكييف comes from a verbal root and can look like a verbal noun, here تكييف الهواء is a fixed noun phrase meaning air conditioning.

So in this context, you should understand it as a thing or system, not as the action conditioning the air.

The natural interpretation is:

• The air conditioning is good.