Learn Polish: Rano przeczytałyśmy polską gazetę. - We read a Polish newspaper in the morning.

Przeczytałyśmy is the perfective form of the verb. In Polish, adding prefixes like prze- often changes a verb from an ongoing action to a completed one. Because we completely finished reading the newspaper this morning, we use the perfective przeczytać instead of the imperfective czytać (which would imply we were in the middle of reading it).

Who exactly is the 'we' in this sentence?

The 'we' refers to a group without any men, like a group of women. You can tell by the ending -łyśmy in przeczytałyśmy. If the group included at least one man, the verb would take the masculine personal ending instead, making it przeczytaliśmy.

Why do the words polska gazeta change to polską gazetę?

The newspaper is the direct object of the sentence (the thing being read), so it takes the Accusative case. For feminine words ending in -a, the noun ending changes to -ę (gazeta becomes gazetę) and the adjective ending changes to -ą (polska becomes polską).

Why isn't there a preposition like 'w' (in) before rano?

Rano is an adverb that already means 'in the morning' all by itself. You do not need to add any extra prepositions in front of it.