Numbers
 

I'm learning how to count in Japanese and I know that
4 is yon / shi and
7 nana / shichi,
9 kyu / ku,
14 juyon / jushi,
17 junana / jushichi,
19 jukyu / juku

but wich one is more common in Japanese society???

it is not a matter of which one is more common because they are used at different times for different things.

for example you would say 四月 "shi gatsu" for April but if you want to say 4 times you say 四回 "yon kai" and you would say 四時 "yo ji" for 4 o'clock (which is not yon, but yo)

九 "kyuu" is used for numbers, like when you say 9, 29 etc. but when you want to say time/month you use 九 "ku" like 九時 "ku ji" and 九月 "ku gatsu"

so it is not a matter of which one is more common, it is a matter of using the correct one in the correct situation. numbers and as an extension of that, counters, are difficult to learn at first but you get the hang of them eventually.

Yeah, that's true. You just have to know both of them, familiarise with them, and know how to differentiate them so that you would know which to use in whichever situations.

Yeah, and don't freak out. When is comes to this stuff, it will just sort of "feel" natural to say kuji instead of kyuji.

You'll also discover the wonderful world of japanese "counters". Things in japanese get counters depending on whether it's people, socks, paper, bowling balls and so on. For example, long cylinder like objects will be counted different than flat ones like a train ticket or piece of paper. Like 2 bottles of beer (cylinder like): biru NI HON.

so, when you are making a nuber like 15, you put the 十 in front of 五　making it 十五？

yep! juugo （十五）