Kereta saya yang rosak menjejaskan perjalanan saya.

A natural breakdown is:

• kereta = car
• saya = my / I / me
• yang rosak = that is broken / which is broken
• menjejaskan = affects / affected / has affected
• perjalanan saya = my journey / my trip

So the structure is roughly:

[My broken car] [affected] [my journey].

More literally:

Car my that broken affected journey my.

Malay often puts modifiers after the noun, so it can look reversed compared with English.

Because in Malay, descriptive information usually comes after the noun.

So:

• kereta merah = red car
• rumah besar = big house
• kereta saya yang rosak = my car that is broken

In English, we usually put many modifiers before the noun: my broken car.
In Malay, that idea is often expressed after the noun: kereta saya yang rosak.

In this sentence, yang links the noun to a description about it. It works a bit like that, which, or who in English relative clauses.

So:

• kereta saya yang rosak = my car that is broken
• lelaki yang tinggi = the man who is tall
• buku yang menarik = the book that is interesting

Here, yang introduces the information rosak and tells us that this description belongs to kereta saya.

Normally, no. That sounds awkward or unclear.

Without yang, the sentence becomes harder to parse, because rosak is no longer clearly attached as a description of kereta saya.

• Kereta saya yang rosak... = my car that is broken...
• Kereta saya rosak. = My car is broken.

So if rosak is being used to describe kereta saya inside a bigger noun phrase, yang is the normal way to connect it.

Malay often does not use a verb like is/are in the way English does.

So:

• Kereta itu besar = That car is big
• Rumah saya jauh = My house is far
• kereta saya yang rosak = my car that is broken

English needs is in that is broken. Malay can simply say yang rosak.

Rosak means broken, damaged, spoiled, or not working properly, depending on context.

With a car, rosak usually means something like:

• broken down
• damaged
• not functioning properly

So kereta saya yang rosak could mean:

• my broken car
• my car that broke down
• my car that is damaged

The exact nuance depends on context.

Menjejaskan is a verb meaning to affect, to disrupt, to impair, or to have a negative effect on.

It is built from a root plus affixes:

• root: jejas
• prefix: meN-
• suffix: -kan

So:

• jejas = affect / disturb / impair
• menjejaskan = to affect something, usually negatively

In this sentence, it means the broken car had a negative effect on the journey.

In normal standard Malay, the affixed form is usually what you use in a full sentence.

• jejas is the root
• menjejaskan is the active transitive verb

Since the subject is doing something to an object:

• Kereta saya yang rosak = subject
• menjejaskan = verb
• perjalanan saya = object

the affixed verb is needed.

This is very common in Malay:

• baca → membaca = read
• guna → menggunakan = use
• jejas → menjejaskan = affect

It is repeated because each noun phrase needs its own possessor if you want to be explicit:

• kereta saya = my car
• perjalanan saya = my journey

So the sentence means:

My broken car affected my journey.

If you remove the second saya:

• Kereta saya yang rosak menjejaskan perjalanan

that means My broken car affected the journey / travel, which is more general and less specific. It no longer clearly means my journey.

It is the same word, but the function changes depending on position.

• Saya penat = I am tired
  Here saya means I.

• kereta saya = my car
  Here saya means my.

Malay does not have a separate possessive form like English I → my.
Instead, the pronoun comes after the noun:

• rumah saya = my house
• kawan saya = my friend
• perjalanan saya = my journey

Because Malay usually places the possessor after the noun.

So:

• kereta saya = my car
• buku dia = his/her book
• rumah kami = our house

This is the normal pattern in Malay:

noun + possessor

That is different from English, which often uses:

possessor + noun

By itself, Malay does not usually mark tense the way English does.

So menjejaskan can mean:

• affects
• is affecting
• affected
• has affected

The exact time is understood from context, or from time words such as:

• semalam = yesterday
• sekarang = now
• esok = tomorrow

For example:

• Semalam, kereta saya yang rosak menjejaskan perjalanan saya.
  = Yesterday, my broken car affected my journey.

Without a time word, the sentence is tense-neutral.

Yes, perjalanan is a good standard word for journey, trip, or travel.

Depending on context, it could refer to:

• a specific trip
• a journey from one place to another
• travel in general

In this sentence, perjalanan saya most naturally means my journey or my trip.

If the context were specifically about a daily commute, other wording might also be possible, but perjalanan saya is perfectly understandable and natural.

It sounds fairly neutral to formal, and it is grammatically standard.

A few points:

• saya is neutral/polite
• menjejaskan is a fairly formal or standard verb
• the whole sentence sounds like good written Malay

In casual speech, people might choose simpler or more conversational wording, but this sentence is correct and natural, especially in writing or careful speech.

In this sentence, it most naturally describes kereta saya.

That is because yang + description usually refers back to the noun phrase immediately before it:

• kereta saya yang rosak = my car that is broken

So the listener understands that rosak applies to the car, not to the journey.

If you wanted to describe the journey instead, the structure would need to be different.