Zgłosiliśmy sprzeciw na zebraniu.

Zgłosiliśmy is the 1st person plural past tense form of zgłosić.

Breakdown:

• zgłosić = the perfective verb, meaning to report / to submit / to raise
• -liśmy = we in the past

So the verb itself already tells you:

• who did it: we
• when: in the past

A useful extra point:
zgłosiliśmy is the form used for a mixed group or a group including at least one male.
If the speakers were all female, it would be zgłosiłyśmy.

Because Polish usually does not need subject pronouns like I, you, we, they unless you want emphasis or contrast.

The ending of zgłosiliśmy already means we, so my is unnecessary.

• Zgłosiliśmy sprzeciw na zebraniu. = normal
• My zgłosiliśmy sprzeciw na zebraniu. = more emphatic, like we were the ones who objected

This is very common in Polish.

This is a question of aspect, which is very important in Polish.

• zgłosić → perfective
• zgłaszać → imperfective

Here, zgłosiliśmy shows a completed action: we raised/lodged the objection.

By contrast, zgłaszaliśmy sprzeciw would suggest something more like:

• we were objecting
• we used to object
• we were in the process of raising objections

In this sentence, the idea is a single completed act, so zgłosiliśmy is the natural choice.

Because zgłosić takes a direct object, and direct objects are usually in the accusative case.

Here:

• sprzeciw is the object of zgłosiliśmy

The reason it looks unchanged is that sprzeciw is a masculine inanimate noun, and in the singular its:

• nominative = sprzeciw
• accusative = sprzeciw

So the form stays the same.

For comparison:

• nominative: sprzeciw
• genitive: sprzeciwu
• instrumental: sprzeciwem

It is a very common and fairly formal Polish expression meaning something like:

• to raise an objection
• to lodge an objection
• to register opposition

It does not just mean casually to disagree.

So:

• Nie zgadzamy się. = We don’t agree.
• Zgłaszamy sprzeciw. = We formally/objectively state opposition.

This phrase is especially common in:

• meetings
• official discussions
• legal or administrative contexts
• voting or procedural situations

Because na can mean different things depending on the case:

• na + accusative = movement to something
• na + locative = being at/on something

Here there is no movement. The action happened at the meeting, so Polish uses:

• na zebraniu = at the meeting

Compare:

• Idziemy na zebranie. = We are going to the meeting.
• Jesteśmy na zebraniu. = We are at the meeting.
• Zgłosiliśmy sprzeciw na zebraniu. = We raised an objection at the meeting.

So zebraniu is the locative singular of zebranie.

Not exactly.

Zebranie usually means a meeting, especially a more organized, official, or structured one:

• a staff meeting
• a committee meeting
• a school-parent meeting
• an association meeting

Spotkanie is broader and often less formal:

• a meeting with a friend
• an appointment
• a get-together
• a business meeting too, depending on context

So in this sentence, zebranie sounds appropriate because zgłosić sprzeciw fits well with a more formal meeting setting.

Yes. Polish word order is much more flexible than English word order.

The sentence:

• Zgłosiliśmy sprzeciw na zebraniu.

is perfectly natural, but other orders are also possible:

• Na zebraniu zgłosiliśmy sprzeciw.
  Emphasizes where/when it happened.

• Sprzeciw zgłosiliśmy na zebraniu.
  Emphasizes the objection itself.

Polish changes word order mainly for:

• emphasis
• topic/focus
• style

So the basic meaning stays the same, but the emphasis changes.

It can suggest either, depending on context.

In many cases, na zebraniu naturally covers both:

• at the meeting
• during the meeting

That is because Polish often uses the same phrase for being present at an event and for something happening in the course of that event.

So in this sentence, English might translate it as either:

• at the meeting
• during the meeting

Both fit the Polish well.