[sothis]

a general rule of thumb is that if there's only a surname, it goes in the first name field. the first name field should never be blank, but the last name field very likely may be.

in easy cases, like John Smith, john goes in first name, smith is in last name - by western definitions i mean what was just written, instead of Smith John

in most other cases, things go in the first name field and last is blank. 'Dr. Jones', 'The Wrestler', 'Smith' (if we dont know the other name), etc

main name should be the legal name, basically, except in rare exceptions. so if someone's name is John Smith and they are commonly called Johnny in the anime, John Smith is still the main name (first line of the names in the submission form), 'Johnny' is added as an additional name, as 'nickname'

there are cases when we don't follow that exactly - for example, you don't find out the last names of the characters in angel beats till later on, so it's kind of spoilery. for that reason, we only list the first names, and the first/last name combos are set to 'searchable'.

i know i gave way more info than you were asking for, i think the simple answer is that if only the last name is known, it should indeed go into the 'first' field. that's mostly because of formatting - last name is automatically capitalized in the page title, etc