وتد الخيمة صغير لكنه قوي.

Because this is a present-tense nominal sentence in Arabic. In Modern Standard Arabic, the verb to be is usually not stated in the present.

So:

• وتد الخيمة صغير = The tent peg is small
• لكنه قوي = but it is strong

If you wanted the past, you would normally use كان:

• كان وتد الخيمة صغيرًا = The tent peg was small

This is an iḍāfa construction, often called the construct phrase. It is the normal Arabic way to say things like:

• the door of the house
• the teacher of the school
• the tent peg

In an iḍāfa:

1. The first noun does not take ال
2. The first noun also does not take tanwīn
3. The second noun is put in the genitive

So:

• وتد الخيمة = the peg of the tent / the tent peg

That is why you do not say الوتد الخيمة here.

Because in an iḍāfa, the definiteness of the whole phrase depends on the second noun.

Here, the second noun is الخيمة = the tent, which is definite because it has ال.

So:

• وتد الخيمة = the tent peg
  even though وتد itself does not have ال

A useful pattern:

• كتابُ طالبٍ = a student's book
• كتابُ الطالبِ = the student's book

So the whole phrase becomes definite when the second part is definite.

Because it is the second noun in an iḍāfa, and that noun is normally in the genitive case.

So in a fully vowelled version, you would have:

• وَتَدُ الخَيْمَةِ

The kasra on الخيمةِ shows the genitive.

This is very common in Arabic:

• بابُ البيتِ = the door of the house
• مفتاحُ السيارةِ = the key of the car
• وتدُ الخيمةِ = the tent peg

Because they are the predicates of the sentence: they tell you something about وتد الخيمة.

Arabic often works like this in nominal sentences:

• subject + predicate

So:

• وتد الخيمة صغير = The tent peg is small
• لكنه قوي = but it is strong

These words are not just sitting next to the noun as direct modifiers in one noun phrase. They are completing the meaning of the sentence.

Because they are predicates, not attributive adjectives.

In Arabic, the predicate in a sentence like this is usually indefinite:

• البيت كبير = The house is big
• الرجل طويل = The man is tall
• وتد الخيمة صغير = The tent peg is small

If you wanted an adjective directly modifying a noun inside a noun phrase, it would match in definiteness too. For example:

• البيت الكبير = the big house

So here:

• وتد الخيمة صغير means The tent peg is small
• not the small tent peg

That difference is important.

لكنه means but it or but he, depending on context.

It is made of:

• لكنّ = but / however
• ـه = him / it (masculine singular pronoun)

In this sentence, the pronoun refers back to وتد, so:

• لكنه قوي = but it is strong

Arabic often uses this attached pronoun instead of repeating the noun:

• وتد الخيمة صغير لكنه قوي
• literally: The tent peg is small, but it is strong

Because وتد is a masculine singular noun, and the pronoun has to agree with it.

So the sentence uses:

• ـه = masculine singular it/he

If the noun were feminine, you would expect a feminine pronoun instead.

This is grammatical gender, not biological gender. Even inanimate things in Arabic are either masculine or feminine, and other words must agree with them.

A fully vowelled version would be:

وَتَدُ الخَيْمَةِ صَغِيرٌ لَكِنَّهُ قَوِيٌّ

This shows several useful grammar points:

• وَتَدُ → nominative, because it is the subject
• الخَيْمَةِ → genitive, because it is the second part of the iḍāfa
• صَغِيرٌ → nominative predicate
• لَكِنَّهُ → but it
• قَوِيٌّ → nominative predicate after لكنّ

Seeing the vowels can make the structure much clearer, especially when you are learning case endings.