Learn Finnish: Minä olin eilen myyjänä. - I was working as a salesperson yesterday.

This is the essive case, which you form by adding -na or -nä to a noun. It is used to talk about a temporary role, position, or state. Here, myyjänä translates to "as a salesperson" or "in the role of a salesperson".

How do I know whether to add -na or -nä?

You use the rule of vowel harmony. Because the word myyjä contains the front vowels y and ä, the ending must also use a front vowel. Therefore, it takes -nä instead of -na.

The English translation says "was working", but the Finnish sentence only uses olin (was). Where is the word for "working"?

You don't need a separate verb for "working" here. In Finnish, combining the verb olla (to be) with a profession in the essive case (like myyjänä) naturally means you are acting or working in that role. Literally, the sentence says "I was as a salesperson".

Can I leave out Minä and just say Olin eilen myyjänä?

Yes, you can! Because the verb olin (I was) already tells us the subject is "I" through its -n ending, dropping the pronoun minä is very common and sounds perfectly natural.