When do I use "ㅇ"?

I see the placeholder when it makes the ng sound but I also see it when it is silent.
For example when writing "안녕하세요" it is used in two ways.
Is there any rule when to use it?

Korean language needs at leaste one vowl and one concernent for

When do I use "ㅇ"?

I see the placeholder when it makes the ng sound but I also see it when it is silent.
For example when writing "안녕하세요" it is used in two ways.
Is there any rule when to use it?

Michaelle is close but no cigar!  Here's what happens. Nothing can start with a vowel.  If you see a vowel by itself, it must have a consonant in front of it.  If it does not, use the ㅇ (iung) consonant.  ㅏ=아, ㅣ=이, ㅛ=요, etc.  This is true with dipthongues also.  ㅇ (iung) is silent here.  What is placeholder?  Is this something fancy for your pencil?

When do I use "ㅇ"?

I see the placeholder when it makes the ng sound but I also see it when it is silent.
For example when writing "안녕하세요" it is used in two ways.
Is there any rule when to use it?

To write Korean hangul, there is at least one vowel and one consonant, 'ㅏ'is a consonant, it has own sound like 'a', but cannot used its alone. So, we need a vowel with 'ㅏ' which has no sound, 'ㅇ'.

'안' is composed one vowel 'ㅇ', one consonant 'ㅏ' and one more vowel, called final consonant 'ㄴ' and all of them make one sound '안' - [an].

And '요' has same rule. the consonant 'ㅛ' can make its own sound 'yo', but cannot be written alone.