Learn Hardcore Indonesian: Saya merasa cepat lelah kalau mengetik terlalu lama di papan ketik yang keras. - I get tired quickly if I type too long on a stiff keyboard.

Merasa adds a nuance of subjective perception / noticing: you’re reporting your own feeling or observation.
Without merasa, the sentence is still correct, just a bit more direct and statement-like.

All of these are grammatically correct; the difference is subtle:

Use saya merasa cepat lelah when you’re emphasizing your feeling/awareness of this tendency.
Use saya cepat lelah as a simple statement of fact or general tendency.
Use saya lelah for your current state: I’m tired (now).

What exactly does cepat lelah mean? Why isn’t it something like lelah dengan cepat?

Cepat lelah literally is quick tired, and it’s a very common pattern in Indonesian:

cepat lelah = get tired quickly / easily tired
cepat marah = easily angered
cepat lapar = get hungry quickly

So cepat before an adjective often means easily / quickly becomes X, not just “fast” in a physical sense.

You could say lelah dengan cepat, but:

It sounds less natural and more “translated” from English.
Everyday Indonesian strongly prefers the pattern cepat + adjective.

So saya cepat lelah is the natural way to say I get tired easily / quickly.

In kalau mengetik terlalu lama, does kalau mean “if” or “when”? Could I use jika or ketika instead?

Kalau can mean both if and when/whenever, depending on context.

Here:

Saya merasa cepat lelah kalau mengetik terlalu lama... = I feel I get tired quickly when I type for too long...
(also understandable as whenever I type for too long).

About the alternatives:

kalau
- Very common, conversational.
- Can be if or when/whenever.
jika
- More formal, mainly if (conditional).
- Saya cepat lelah jika mengetik terlalu lama sounds like a formal written style.
ketika
- Mainly when (at the time that).
- Saya cepat lelah ketika mengetik terlalu lama is okay, but it sounds a bit more like a specific time frame rather than a general habit.

In everyday speech, kalau is the most natural here.

Why is it mengetik, not just ketik or something like mengketik?

The base word is ketik (to type). To make an active verb, Indonesian adds the meN- prefix:

meN- + ketik → mengetik (to type)

A few points:

mengetik is the standard active verb form:
- Saya sedang mengetik. = I am typing.
ketik by itself is the base/root form. You’ll see it:
- in dictionaries,
- in commands: Ketik nama Anda di sini. = Type your name here.

You should not say mengketik; that form is incorrect. Just memorize mengetik as the correct verb from ketik.

What does terlalu lama mean exactly? Is it just “for a long time” or “for too long”? Could I say it some other way?

Terlalu means too / excessively, and lama means long (in time).

terlalu lama = too long / for too long, with the idea of excess.

Examples:

menunggu terlalu lama = wait too long
mandi terlalu lama = shower for too long

If you only want to say “for a long time” (neutral, no “too”), you’d use lama without terlalu:

kalau mengetik lama = when I type for a long time (not necessarily too long).

Other colloquial options:

kelamaan mengetik = spending too long typing (more casual).
kalau saya kelamaan mengetik ≈ kalau saya mengetik terlalu lama.

Why is it di papan ketik and not something like dengan papan ketik or just papan ketik?

Di is a preposition meaning at / in / on (location). Here:

di papan ketik = on the keyboard (literally: at the keyboard).

This matches the common pattern in Indonesian:

mengetik di komputer = type on the computer
mengetik di laptop = type on the laptop
mengetik di HP = type on the phone

Using dengan (with/by means of) would sound odd here:

mengetik dengan papan ketik is grammatically understandable (type with a keyboard), but it’s not natural everyday phrasing for this idea.

You could omit the preposition in some contexts (e.g. papan ketik keras capek dipakai), but in your sentence, di papan ketik is the normal and clear choice.

What exactly is papan ketik? Do people really say that, or do they just say keyboard?

Papan ketik literally means typing board and is the standard Indonesian term for keyboard.

Usage:

In formal writing, textbooks, or official language, papan ketik is common.
In everyday conversation, many people simply say keyboard (the English loanword), especially in tech contexts.

So these are both understandable:

mengetik di papan ketik (more “Indonesian-sounding” / formal-neutral)
mengetik di keyboard (very common in daily speech)

Why is it papan ketik yang keras and not just papan ketik keras? What does yang do here?

Yang introduces a relative clause or a descriptive phrase about a noun. It’s similar to that / which / who in English, but it’s also used more broadly to link a noun and its description.

Compare:

papan ketik keras
- a hard keyboard (adjective directly after noun)
- fine, and still correct.
papan ketik yang keras
- a keyboard that is hard / the keyboard which is hard
- sounds a bit more like you’re identifying or specifying that particular type of keyboard.

In your sentence, yang keras helps to:

clearly attach keras specifically to papan ketik, and
make it sound like a distinctive characteristic: keyboards that are hard (to press / loud / stiff), not just any keyboard.

Both papan ketik keras and papan ketik yang keras are grammatical; yang adds a slight feeling of “the ones which are hard”.

What kind of “hard” is keras here? Does it mean physically hard, or loud, or something else?

Keras is quite flexible; it can mean:

hard / firm (texture)
loud (sound)
strong / intense / strict

In the context of papan ketik yang keras, it typically implies:

keys that are stiff / require a lot of force, and/or
keys that make a loud clicking sound.

So the idea is: a keyboard that is uncomfortable or tiring to type on because it feels hard or loud, not just “made of hard material.”

What’s the difference between lelah, capek, and letih? Could I say Saya merasa cepat capek instead?

All three relate to being tired, but they differ in tone/usage:

lelah
- Neutral to formal.
- Common in written language, polite speech, medical or work contexts.
capek / capai
- Very common in everyday, informal speech.
- Capek is more colloquial than lelah.
letih
- More literary or poetic.
- You might see it in written narratives or formal expressions.

In your sentence, you can absolutely say:

Saya merasa cepat capek kalau mengetik terlalu lama di papan ketik yang keras.

That will sound more casual / conversational than cepat lelah. The original with lelah is slightly more neutral/formal.