Saya turun tangga perlahan kerana lutut saya masih sakit.

Malay verbs usually do not change form for tense the way English verbs do. The verb turun stays turun.

So the time is normally understood from:

• context
• time words such as semalam (yesterday), sekarang (now), esok (tomorrow)
• the overall situation

In this sentence, without extra time words, it most naturally sounds like a present/current situation:

• Saya turun tangga perlahan kerana lutut saya masih sakit.
• I go/am going down the stairs slowly because my knee still hurts.

If you wanted to make the time clearer, you could add something like:

• Sekarang saya turun tangga perlahan... = Now I’m going down the stairs slowly...
• Semalam saya turun tangga perlahan... = Yesterday I went down the stairs slowly...

Turun already means go down / descend, so adding ke bawah is often unnecessary.

• turun tangga = go down the stairs / descend the stairs

About the other options:

• turun ke bawah = go downwards / go down below
  This is possible, but with tangga it can sound redundant because turun already contains the idea of downward movement.

• menuruni tangga = descend the stairs
  This is also correct, but it is more formal and more written-style. In everyday speech, turun tangga is very natural.

So:

• Saya turun tangga = normal everyday Malay
• Saya menuruni tangga = correct, but more formal/literary

Tangga can refer to:

• a stair
• stairs
• a staircase

Malay often does not force you to mark singular vs plural the way English does. The exact meaning comes from context.

So in this sentence, turun tangga is most naturally understood as:

• go down the stairs
• or go down the staircase

If you specifically wanted steps or a flight of stairs, context or extra words would help, but tangga by itself is very normal here.

Both are possible.

• perlahan = slow / slowly
• perlahan-lahan = slowly, often with a more natural adverbial feel, sometimes slightly more emphatic or gentle

So these are both fine:

• Saya turun tangga perlahan...
• Saya turun tangga perlahan-lahan...

The version with reduplication, perlahan-lahan, often sounds a bit more explicitly adverbial in everyday Malay, but perlahan is also very common and correct.

Very roughly:

• perlahan = concise, straightforward
• perlahan-lahan = a bit more descriptive, like slowly / little by little / carefully slowly, depending on context

In Malay, adverbs like perlahan commonly come after the verb or verb phrase.

So:

• turun tangga perlahan = go down the stairs slowly

That is the most natural basic order here.

Malay word order is often:

• Subject + Verb + Object/Complement + Adverb

So this sentence follows a very common pattern:

• Saya = subject
• turun tangga = verb phrase
• perlahan = adverb
• kerana... = reason clause

You may sometimes see other orders for emphasis, but the version in your sentence is standard and natural.

Yes, you can.

• Saya turun tangga perlahan...
• Saya turun tangga dengan perlahan...

Both mean essentially the same thing.

However, the version without dengan is usually more natural and concise in ordinary Malay.
Using dengan perlahan can sound a bit more explicit or slightly more formal.

So for everyday speech, perlahan alone is often preferred.

Both can mean because.

• kerana = more standard, slightly more formal, common in writing
• sebab = very common in speech, more everyday and conversational

So these are both possible:

• Saya turun tangga perlahan kerana lutut saya masih sakit.
• Saya turun tangga perlahan sebab lutut saya masih sakit.

The meaning is the same.
If you are aiming for neutral standard Malay, kerana is an excellent choice.

In Malay, possession is usually shown by putting the possessor after the noun.

So:

• lutut saya = my knee
• rumah saya = my house
• buku saya = my book

This is the normal pattern:

• noun + pronoun/person

So saya lutut would be incorrect.

The first saya is the subject:

• Saya turun tangga... = I go down the stairs...

The second saya shows possession:

• lutut saya = my knee

So they do different jobs.

You sometimes can omit a possessor if it is obvious from context, but in a sentence like this, keeping saya is the clearest and most natural choice:

• lutut saya masih sakit = my knee still hurts

Without it, lutut masih sakit could sound less specific, or depend heavily on context.

By itself, lutut can mean:

• knee
• knees

Malay often leaves number unmarked unless it needs to be made clear.

So lutut saya masih sakit could mean:

• my knee still hurts
• my knees still hurt

Context tells you which one is meant.

If you want to be specific, you can say:

• lutut kanan saya = my right knee
• lutut kiri saya = my left knee
• kedua-dua lutut saya = both my knees

Masih means still.

So:

• lutut saya sakit = my knee hurts / is sore
• lutut saya masih sakit = my knee still hurts / is still sore

It shows that the condition is continuing.

This is a very common word in Malay for ongoing states:

• Dia masih tidur. = He/She is still sleeping.
• Saya masih di sini. = I’m still here.

Sometimes, yes, but masih is clearer and more standard here.

• masih = still
• lagi can mean again, more, or sometimes still depending on context

So:

• lutut saya masih sakit = clear, standard: my knee still hurts
• lutut saya sakit lagi could be understood differently depending on context, such as my knee hurts again or in some contexts still hurts

Because lagi has more than one possible meaning, masih is the better choice in this sentence.

It can mean either, depending on what it refers to.

With a person, sakit often means:

• ill
• sick

Example:

• Saya sakit. = I am sick / unwell.

With a body part, sakit usually means:

• painful
• sore
• hurts

So:

• lutut saya sakit = my knee hurts / my knee is sore

That is why in your sentence, sakit is understood as physical pain in the knee, not illness.

Yes, it is natural and correct.

A native speaker might also say:

• Saya turun tangga perlahan-lahan kerana lutut saya masih sakit.
• Saya turun tangga dengan perlahan sebab lutut saya masih sakit.

These are just small style differences:

• kerana vs sebab
• perlahan vs perlahan-lahan
• with or without dengan

Your original sentence is perfectly good standard Malay.