When and How to use ’ー’　in katakana? たとえば、リーダー、メッセージ、タクシー？i know it shoud b pronunced longer,but HOW and WHEN?
when and how to use ’ー’　in katakana?
たとえば、リーダー、メッセージ、タクシー？i know it should be pronunced longer, but HOW and WHEN to use it? can i simply use it as i wish...or does it have any specific rules for using 'ー'？for example, can i write 'leader' in リーダー、without 'ー'　so, can it just be リダ? It is really hard for me to ask this, i have asked this plenty of times, but seems like i still don't get the answer, (or maybe it is my mistake for not knowing what and how to ask) じゃ、anyone...よろしくお願いします！！ありがとう！

Question

Answer

hi, You can't write リダ　for リーダー

writer is also ライター　　

actually there is no rule when you use,

I think "er" and "or"sounds uses "ー"

for example, "computer, コンピューター"  "social worker:ソーシャルワーカー" etc..

Answer

Yeah, there's no real rule, but you can think of it as a marker that lengthens the vowel sound.
For example, リーダー would be りいだあ in hiragana (never written in this way, just an example). This makes it pretty clear that the i's and a's are longer than the consonants. So, the katakana ー makes the vowel sound longer.

That's it. There's no "how" or "when". Words are words, you can't really change them.
I don't really know how to explain this. You can't write "リダ" for リーダー because you can't write "ldr" for leader.