Mein Bruder sucht lieber einen unbefristeten Vertrag, weil ihm ein befristeter zu unsicher ist.

Here, lieber means preferably or rather.

It comes from gern / gerne, which is often used in German to express liking or preference with verbs.

• Ich esse gern Pizza. = I like eating pizza.
• Ich esse lieber Pizza. = I’d rather eat pizza.

So:

• Mein Bruder sucht lieber einen unbefristeten Vertrag
  means that he would rather look for / prefers an open-ended contract.

It is not exactly the same as more in English. It specifically expresses preference.

Because suchen takes a direct object, and the direct object is in the accusative case.

The noun is:

• der Vertrag = the contract

In the accusative, der becomes einen when used with the masculine indefinite article:

• nominative: ein unbefristeter Vertrag
• accusative: einen unbefristeten Vertrag

So after sucht, we need the accusative form:

• Mein Bruder sucht einen unbefristeten Vertrag.

That is because the adjective is describing a masculine accusative noun after einen.

The phrase is:

• einen unbefristeten Vertrag

German adjective endings depend on:

1. the gender
2. the case
3. the article

Here:

• Vertrag is masculine
• it is in the accusative
• it has the indefinite article einen

So the adjective takes the ending -en:

• einen unbefristeten Vertrag

This is a standard mixed-declension pattern.

These are opposite adjectives:

• befristet = fixed-term, temporary, limited in duration
• unbefristet = permanent, open-ended, not limited in duration

In job and contract contexts:

• ein befristeter Vertrag = a fixed-term contract
• ein unbefristeter Vertrag = a permanent / open-ended contract

The prefix un- often makes a word negative or opposite, similar to un- in English words like unclear.

Because German uses the dative here.

The pattern is:

• etwas ist jemandem zu + adjective

This means:

• something is too something for someone

So:

• ihm = to him / for him
• ein befristeter = a fixed-term one
• zu unsicher = too insecure / too uncertain

So the structure is:

• weil ihm ein befristeter zu unsicher ist
• literally: because a fixed-term one is too insecure for him

That is why ihm is dative, not nominative (er) or accusative (ihn).

Because the noun Vertrag is being left out to avoid repetition.

Earlier, the sentence already said:

• einen unbefristeten Vertrag

So in the second half, German can omit Vertrag and let the adjective phrase stand in for it:

• ein befristeter = a fixed-term one
• understood: ein befristeter Vertrag

This is very common in German and English:

• I want the red shirt, not the blue one.
• Ich will das rote Hemd, nicht das blaue.

So here:

• ein befristeter
  means a fixed-term contract, with Vertrag understood.

Because in the second clause, that phrase is the subject, not the direct object.

The clause is:

• weil ihm ein befristeter zu unsicher ist

The subject is ein befristeter = a fixed-term one

Subjects are in the nominative case.

So we get:

• nominative masculine: ein befristeter
• accusative masculine: einen befristeten

Compare:

• Er sucht einen befristeten Vertrag.
  direct object → accusative

• Ein befristeter Vertrag ist ihm zu unsicher.
  subject → nominative

That is exactly the difference between the two parts of the sentence.

Because weil introduces a subordinate clause, and in German subordinate clauses, the conjugated verb normally goes to the end.

Main clause:

• Mein Bruder sucht lieber einen unbefristeten Vertrag

Subordinate clause:

• weil ihm ein befristeter zu unsicher ist

The verb in that subordinate clause is ist, and it comes at the end because of weil.

This is one of the most important German word-order rules:

• Er bleibt zu Hause, weil er krank ist.
• Ich lerne Deutsch, weil es interessant ist.

Same pattern here.

No. Here, zu means too, not to.

• unsicher = insecure, uncertain, unstable
• zu unsicher = too insecure / too uncertain

So:

• ein befristeter ist ihm zu unsicher means
• a fixed-term one is too uncertain for him

This zu + adjective pattern is very common:

• zu teuer = too expensive
• zu schwer = too difficult / too heavy
• zu spät = too late

In this sentence, unsicher is best understood as something like:

• uncertain
• not secure
• too unstable

With contracts or jobs, unsicher often refers to a lack of security or stability rather than physical danger.

So ein befristeter Vertrag ist zu unsicher suggests:

• it does not feel stable enough
• it does not offer enough long-term security

So although unsicher can sometimes mean insecure or unsafe, in this context uncertain / not secure enough is the best sense.

Yes, absolutely.

That would be the full, explicit version:

• weil ihm ein befristeter Vertrag zu unsicher ist

The original sentence leaves out Vertrag because it is already understood from earlier in the sentence. Both versions are correct.

The shorter version is more natural because it avoids repetition:

• ... einen unbefristeten Vertrag, weil ihm ein befristeter zu unsicher ist.

It has two parts:

1. Main clause
   Mein Bruder sucht lieber einen unbefristeten Vertrag

2. Subordinate clause introduced by weil
   weil ihm ein befristeter zu unsicher ist

A useful structural breakdown is:

• Mein Bruder = subject
• sucht = verb
• lieber = expresses preference
• einen unbefristeten Vertrag = direct object

Then:

• weil = because
• ihm = dative person affected
• ein befristeter = subject of the subordinate clause
• zu unsicher = predicate adjective phrase
• ist = verb at the end

So the grammar is very typical German:

• main clause: verb in second position
• subordinate clause with weil: verb at the end