when to write buta and but in iupac nomenclature

Dear student,
Here is the answer of your asked query:

An extra "a" is added to the word root if the primary suffix to be added to the word root begins with a consonant. 

Here an extra "a" has been added to the word root since the primary suffix i.e. diene or diyne begins with consonant i.e. 'd' instead of a vowel as is the case in first three cases.

Regards