Był miły wobec sprzedawcy.

Polish often drops subject pronouns when they are clear from the verb form.

Here, był is the past tense, masculine singular form of być (to be), so in context it usually means he was.

You could say On był miły wobec sprzedawcy, but on would normally be added only for emphasis, contrast, or clarity.

Był is the masculine singular past tense form of być (to be).

Some related forms are:

• była = she was
• było = it was
• byli = they were (masculine personal)
• były = they were (non-masculine-personal)

So był miły tells you the subject is masculine singular.

Because here miły is a predicate adjective after być, and in this kind of sentence Polish normally uses the nominative form of the adjective.

So:

• On był miły. = He was nice.
• Ona była miła. = She was nice.

You may also see instrumental forms like miłym, but usually when the adjective is part of a larger noun phrase, for example:

• Był miłym człowiekiem. = He was a nice person.

In your sentence, the simple adjective after był stays nominative: miły.

Wobec means something like toward, towards, or in relation to.

In this sentence, it shows the person toward whom the behavior was directed: the salesperson.

A useful thing to remember is that wobec is a little more formal or bookish than some other options. With miły, everyday Polish often uses dla:

• Był miły dla sprzedawcy.

Your original sentence is still grammatical and understandable, but wobec can sound a bit more formal.

Because wobec requires the genitive case, and the genitive singular of sprzedawca is sprzedawcy.

So:

• dictionary form: sprzedawca
• after wobec: wobec sprzedawcy

This is very common in Polish: prepositions often force a particular case.

Yes, in this sentence it is genitive, because wobec always takes the genitive.

It is true that sprzedawcy can represent more than one grammatical form in Polish, for example:

• genitive singular
• dative singular
• locative singular
• nominative plural

But the preposition tells you which one it is. After wobec, it must be genitive singular.

Most naturally, yes: sprzedawca is the masculine form, so this suggests a male salesperson.

If you want to refer specifically to a female salesperson, Polish often uses sprzedawczyni:

• Był miły wobec sprzedawczyni.

That said, masculine job titles can sometimes be used more generally in certain contexts, but in ordinary speech sprzedawca usually points to a man.

No. Polish word order is fairly flexible because the endings carry a lot of grammatical information.

The neutral order here is:

• Był miły wobec sprzedawcy.

But you could reorder it for emphasis, for example:

• Wobec sprzedawcy był miły.
  This puts more focus on the salesperson.

The meaning stays very similar, but the emphasis changes. For a learner, the original order is a good, natural default.