Learn Polish: Rząd ogłosił nowy plan. - The government announced a new plan.

The verb ogłosił takes a direct object, which requires the accusative case. Because plan is a masculine inanimate noun, its accusative form is exactly the same as its nominative (dictionary) form. You only use the -ego and -a endings for masculine animate nouns, like in the phrase widzę nowego psa (I see a new dog).

Is ogłosił a perfective or imperfective verb?

It is a perfective verb, meaning the action is fully completed: the announcement was made. If you wanted to say the government was announcing or used to announce plans, you would use the imperfective form ogłaszał. The infinitive verb pair is ogłaszać (imperfective) and ogłosić (perfective).

I have seen the word rząd used in a different context. Does it have other meanings?

Yes, rząd is a very common homonym in Polish. Besides meaning government, it is also the standard word for a row or line of things. For example, you might look for your seat in the fifth row at a cinema by searching for piąty rząd.

How does the noun głos (voice) relate to the verb ogłosił?

The noun głos is the root of the verb ogłosić. By adding the prefix o- (which often indicates encompassing or spreading), you get a verb that literally means to voice something out to the public. Spotting this connection makes it much easier to remember Polish verbs related to speaking, loud noises, and broadcasting!