Hello in arabic there is only consonants. The vowels are replaced by the "harakat". Fat7a damma kassra soukoun. Which we put above each letter to give it a sound

You are right, in standard Arabic it's pronounced Marhaban because we have harakat and this word have this haraka at the last letter اً ، we name this haraka o the letter Tanween and it's pronounced as " an" . I hope this explanation is clear.

i see you new more about harakat down there i hope you understood them well if you need more i can help it's (shuk-ran) if you insist on breaking it but in arabic you shouldn't break words into syllables just follow letters and harkat and this will be easier by practice

Yes, I've got that one. What about the missing vowels between "mrhb"? Oh, is 'shukran' 3 syllables? shu-kri-an? I've been saying shu-key-an? I saw 'shukran' before but I read it as shu-kran.

in standard arabic when the work ends with (ا) we add this mark (اً) we call it tanween and it's equal in pronouncing to (ن) N letter added to the end of the word so marhaba becomes marhaban in standard arabic and by the way thanks is pronounced (shukran) not (shukian)