Od szóstej rano czekam na autobus.

Because od normally requires the genitive case in Polish.

The basic dictionary form is szósta = six o’clock (literally the sixth [hour]).
After od, it changes to szóstej:

• szósta → nominative
• szóstej → genitive

So od szóstej means since six or from six.

Here, od means since.

In this sentence, it shows the starting point of an action that began earlier and is still continuing:

• Od szóstej rano czekam... = I have been waiting since six in the morning...

Polish often uses od + a time expression to mark where something started.

Rano means in the morning.

Without it, od szóstej could mean since six, but it would not be clear whether that is 6 a.m. or 6 p.m. Adding rano makes it specific:

• od szóstej rano = since six in the morning

If the context already makes that obvious, Polish speakers may leave rano out.

Because Polish does not have a separate verb form for the English present continuous.

So czekam can mean both:

• I wait
• I am waiting

In this sentence, because of od szóstej rano, the meaning is clearly ongoing:
I have been waiting since six in the morning.

That is very normal in Polish: the present tense often covers meanings that English expresses with am waiting or have been waiting.

Yes. This is one of the most important differences from English.

Polish often uses the present tense for an action that:

1. started in the past, and
2. is still continuing now.

So:

• Od szóstej rano czekam na autobus.

is natural Polish for:

• I have been waiting for the bus since six in the morning.

English uses the present perfect continuous, but Polish usually just uses the present tense plus a time expression like od szóstej rano.

Because the verb czekać is used with the preposition na when you mean to wait for someone or something.

So:

• czekać na kogoś/coś = to wait for someone/something

Examples:

• Czekam na autobus. = I’m waiting for the bus.
• Czekam na mamę. = I’m waiting for my mum.

You usually cannot just say czekam autobus for this meaning.

It is in the accusative singular, because na after czekać takes the accusative.

The useful thing is that for many masculine inanimate nouns, the accusative singular looks exactly the same as the nominative singular.

So:

• nominative: autobus
• accusative: autobus

That is why the form does not change.

Compare with an animate masculine noun, where you would see a change:

• Czekam na kolegę. = I’m waiting for my friend.

Yes, czekać is an imperfective verb.

That fits because the action is:

• ongoing,
• unfinished,
• extended over time.

Waiting is exactly the kind of action Polish normally expresses with an imperfective verb in a sentence like this.

Using an imperfective verb here matches the idea of I’ve been waiting rather than a completed event.

Yes, absolutely.

Both are grammatical:

• Od szóstej rano czekam na autobus.
• Czekam na autobus od szóstej rano.

The difference is mostly emphasis.

• Od szóstej rano... puts the time phrase first, so it emphasizes how long the waiting has been going on.
• Czekam na autobus... starts with the action itself and sounds a bit more neutral.

Putting od szóstej rano first can sound slightly more dramatic or complaining, depending on context.

There is an implied idea of hour.

Polish tells time using ordinal numbers, so szósta literally relates to the sixth hour. After od, it becomes szóstej.

So in practice:

• o szóstej = at six
• od szóstej = since six

You do not need to add a separate word for hour in normal speech.

Yes, very easily.

Starting with Od szóstej rano... strongly highlights the duration, so it often suggests impatience, annoyance, or frustration:

• Od szóstej rano czekam na autobus.
  = I’ve been waiting for the bus since six in the morning.

Depending on tone, it can sound like: Can you believe how long this has been taking?

So grammatically it is neutral, but in real conversation it often carries emotional emphasis.