كفاءة الطاقة مهمة.

Because Arabic often uses a nominal sentence in the present tense.

كفاءة الطاقة مهمة literally looks like:

• كفاءة الطاقة = energy efficiency
• مهمة = important

So Arabic expresses Energy efficiency is important without an explicit present-tense verb.

If you wanted past or future, Arabic would usually add something else, for example:

• كانت كفاءة الطاقة مهمة = Energy efficiency was important.
• ستكون كفاءة الطاقة مهمة = Energy efficiency will be important.

Yes. كفاءة الطاقة is an iḍāfa (إضافة), often called a genitive construction or construct phrase.

It is made of:

• كفاءة = efficiency
• الطاقة = the energy

Literally, the structure is something like the efficiency of the energy, but in natural English it means energy efficiency.

In an iḍāfa:

1. the first noun is the main thing being described
2. the second noun specifies it

So here, كفاءة is the head noun, and الطاقة tells us what kind of efficiency.

This is normal in an iḍāfa.

In كفاءة الطاقة:

• the first noun (كفاءة) usually does not take الـ
• the second noun (الطاقة) can be definite with الـ

The whole phrase becomes definite because of the second noun.

So كفاءة الطاقة is a definite phrase meaning energy efficiency.

This is one of the key rules of iḍāfa:

• first noun: no الـ
• second noun: can be definite, indefinite, or a pronoun, depending on the meaning

Because كفاءة is a feminine noun, and the predicate adjective agrees with it in gender.

So:

• كفاءة ... مهمة = efficiency ... is important
• موضوع ... مهم = a topic ... is important

Even though energy itself can affect the meaning of the phrase, the grammatical head of كفاءة الطاقة is كفاءة, and that noun is feminine. That is why مهمة is used.

A big clue is the ending ة (tāʾ marbūṭa), which very often marks feminine nouns.

So:

• كفاءة is feminine
• therefore the adjective/predicate is feminine: مهمة

This ending does not make every single noun feminine in all cases, but it is one of the strongest and most common signs.

A careful pronunciation is approximately:

• كفاءة = kafāʾa
• الطاقة = aṭ-ṭāqa
• مهمة = muhimma

A few useful details:

• ء is hamza, a glottal stop, like a brief catch in the throat.
• The final ة in pause is usually pronounced like -a.
• If the word is followed by grammatical endings in full formal reading, ة can sound like t before the ending.

So in normal pausal pronunciation:

• كفاءة sounds like kafāʾa
• مهمة sounds like muhimma

Because ط is a sun letter.

When الـ comes before a sun letter, the l sound assimilates into the next consonant. So:

• written: الطاقة
• pronounced: aṭ-ṭāqa

You still write الـ, but you pronounce a doubled ṭ sound.

Because here مهمة is the predicate of the sentence, and in Arabic that predicate is often indefinite.

So:

• كفاءة الطاقة = the topic / subject
• مهمة = important

This is very common in Arabic nominal sentences:

• الكتاب جديد = The book is new.
• المدينة كبيرة = The city is big.

The adjective-like predicate often appears without الـ.

In fully vocalized Classical or very careful MSA, it would be:

كفاءةُ الطاقةِ مهمّةٌ

Here is why:

• كفاءةُ: nominative, because it is the subject/topic of the nominal sentence
• الطاقةِ: genitive, because it is the second term of the iḍāfa
• مهمّةٌ: nominative, because it is the predicate

Also notice something important:

• كفاءة does not take tanwīn here, because the first noun in an iḍāfa does not take tanwīn.

In everyday writing, these short vowels are usually omitted.

Structurally, yes, that is close to the literal grammar. But in normal English, the natural meaning is Energy efficiency is important.

Arabic iḍāfa often corresponds to English of phrases, but very often English prefers a compound noun instead.

So:

• literal structure: efficiency of energy
• natural English meaning: energy efficiency

Yes, but كفاءة الطاقة مهمة is the normal, straightforward order.

This order is:

• topic / subject first
• predicate second

You may sometimes see the predicate first for style, emphasis, or a different information structure, but for learners the standard pattern to remember is:

• الموضوع + الخبر
• كفاءة الطاقة مهمة

So this sentence is a good model for a basic Arabic nominal sentence.

Functionally, it is an adjective in meaning, but in this sentence it serves as the predicate (khabar) of the nominal sentence.

So grammatically you can think:

• كفاءة الطاقة = the subject/topic
• مهمة = the predicate meaning important

Arabic does this very often with adjectives:

• الجو جميل = The weather is جميل / beautiful
• الدرس سهل = The lesson is easy
• كفاءة الطاقة مهمة = Energy efficiency is important

مهمة comes from the root ه م م in the adjective مهم = important.

The feminine form is:

• مهم = important (masculine)
• مهمة = important (feminine)

A very useful related noun is:

• مهمة = task / mission

So the written form مهمة can mean either:

• important (feminine adjective), or
• task / mission (noun)

In كفاءة الطاقة مهمة, it clearly means important because of the sentence structure.