Post cenam omnes de via disputant; ego autem opinor gelu cras minus futurum esse.

Because post is a preposition that takes the accusative when it means after.
So:

• cena = dinner, supper
• cenam = accusative singular
• post cenam = after dinner

This is just the normal construction after post.

Omnes literally means all or all people. Here it is being used substantively, meaning everyone.

Grammatically, it is nominative plural, the subject of disputant. Latin often uses a plural expression where English might prefer a singular collective word like everyone.

So:

• omnes disputant = everyone is discussing / all are discussing

Here omnes is nominative plural, because it is the subject of disputant (they discuss).

The verb disputant is 3rd person plural, so it needs a plural subject. Omnes fits that perfectly.

Although omnes can also be accusative plural in other contexts, here the sentence structure makes nominative plural the correct interpretation.

Because de meaning about, concerning takes the ablative.

So:

• via = road, way, route
• via here is ablative singular
• de via = about the road/route/journey

This is a very common pattern in Latin: disputare de aliqua re = to discuss something.

Not necessarily. Disputare can mean to discuss, debate, argue about, or examine a topic. It does not have to suggest an angry argument.

So in this sentence, omnes de via disputant is most naturally something like everyone is discussing the route rather than everyone is fighting about it.

Yes, opinor already includes the idea I in its ending. Latin does not need ego unless the speaker wants emphasis or contrast.

Here ego is probably used for contrast with omnes:

• omnes ...; ego autem ...
• everyone ...; but I ...

So ego adds emphasis: as for me, I, however...

Because autem is usually postpositive, which means it normally comes after the first word or phrase of its clause.

So Latin prefers:

• ego autem opinor

rather than putting autem first.

It often means however, but, or on the other hand.

Because opinor is a deponent verb. Deponent verbs have passive forms but active meanings.

So:

• opinor looks like a passive form
• but it means I think, I suppose, I believe

This is something English speakers often have to get used to in Latin.

After verbs of thinking, saying, knowing, and perceiving, Latin often uses an indirect statement construction instead of a separate word meaning that.

English says:

• I think that ...

Latin usually says:

• I think [something] to be ...

In this sentence, the indirect statement is:

• gelu cras minus futurum esse

That whole phrase is the thing the speaker thinks.

In an indirect statement, the subject normally goes into the accusative. So gelu is functioning as the accusative subject of futurum esse.

But gelu is a neuter noun, and in neuter nouns the nominative and accusative are often identical in form. So it looks the same.

In other words, gelu is doing the accusative job here, even though its form does not visibly change.

Because Latin uses the future infinitive in indirect statement when the action is future from the point of view of the main verb.

So after opinor (I think), Latin says:

• gelu ... futurum esse
• that there will be frost / that frost will be...

Futurum esse is the future infinitive of sum.
It is built from:

• futurum = future participle, neuter singular here
• esse = to be

It is neuter singular because it agrees with gelu, which is neuter singular.

Minus means less. Here it is used adverbially, giving the sense that the frost will be less in amount or intensity.

So:

• gelu cras minus futurum esse

means something like:

• that there will be less frost tomorrow
• or more naturally in English, that it will be less frosty/cold tomorrow

Latin is expressing the idea a bit differently from idiomatic English.

Cras means tomorrow, and it belongs to the indirect statement:

• gelu cras minus futurum esse

So the speaker thinks about tomorrow that the frost will be less.

Latin word order is flexible, so cras can stand in a natural place inside the clause without needing to come at the very beginning or the very end.

The grammar matters more than the exact order, but the order does help show emphasis and contrast.

A rough breakdown is:

• Post cenam = setting the time
• omnes de via disputant = what everyone is doing
• ego autem = strong contrast: I, however
• opinor = main verb of the second clause
• gelu cras minus futurum esse = what I think

So the sentence is arranged to contrast everyone else's discussion with my own opinion.