Wczoraj czytałam raport przez dwie godziny, ale przeczytałam tylko połowę.

This is a very common Polish aspect contrast.

• czytałam is imperfective: it focuses on the process, the activity of reading.
• przeczytałam is perfective: it focuses on reaching the end, completing the reading.

So in this sentence:

• czytałam raport przez dwie godziny = I was reading the report for two hours.
• przeczytałam tylko połowę = I only managed to read half of it / I completed only half.

The prefix prze- often helps create a perfective verb from czytać:

• czytać = to read, be reading
• przeczytać = to read through, finish reading

Polish often uses this imperfective + perfective contrast where English might just use read in both parts.

The ending -am shows that the speaker is female and the verb is in the past tense, first person singular.

So:

• czytałam / przeczytałam = I (female) was reading / read
• czytałem / przeczytałem = I (male) was reading / read

Polish past tense agrees with the gender of the speaker in the singular.

Because raport is the direct object of czytałam, so it is in the accusative case.

For many masculine inanimate nouns, the accusative singular looks the same as the nominative singular:

• nominative: raport
• accusative: raport

So:

• czytam raport = I am reading a report
• mam raport = I have a report

But with some other noun types, the accusative would look different.

In Polish, przez + accusative is a common way to express duration.

So:

• przez godzinę = for an hour
• przez dwie godziny = for two hours
• przez tydzień = for a week

Here:

• dwie is the form of two used with feminine nouns
• godziny is the accusative plural form here

Since godzina is feminine, you say:

• jedną godzinę
• dwie godziny

not dwa godziny.

Because godzina is a feminine noun.

With the number 2, Polish distinguishes:

• dwa for many masculine and neuter nouns
• dwie for feminine nouns

Examples:

• dwa domy = two houses
• dwa okna = two windows
• dwie książki = two books
• dwie godziny = two hours

So godzina requires dwie.

Because przez dwie godziny describes how long the activity lasted, and that naturally matches the imperfective verb czytałam.

• czytałam przez dwie godziny = I spent two hours reading

If you use the perfective verb, Polish usually shifts the meaning toward completion within a time limit, often with w instead of przez:

• Przeczytałam raport w dwie godziny = I finished reading the report in two hours

So the contrast is:

• przez dwie godziny = for two hours, duration of an ongoing activity
• w dwie godziny = in two hours, time needed to complete something

That distinction is very important in Polish.

Because połowę is the accusative form, and here it is the direct object of przeczytałam.

• nominative: połowa = half
• accusative: połowę = half (as an object)

So:

• Połowa raportu jest gotowa. = Half of the report is ready.
• Przeczytałam połowę. = I read half.

It could. The full version would be:

• ...ale przeczytałam tylko połowę raportu.

That means exactly ...but I read only half of the report.

In your sentence, raportu is omitted because it is already obvious from the earlier word raport. Polish often leaves out repeated information when the meaning is clear.

So both are possible:

• przeczytałam tylko połowę = I read only half
• przeczytałam tylko połowę raportu = I read only half of the report

Because after połowa / połowę, the thing being divided is usually put in the genitive.

So:

• połowa raportu = half of the report
• połowa książki = half of the book
• połowa chleba = half of the bread

In the sentence:

• przeczytałam połowę raportu

połowę is accusative because it is the object of the verb, and raportu is genitive because it depends on połowę.

Tylko means only, and here it modifies połowę.

So the idea is:

• I read only half

It limits the amount completed. The contrast is important:

• I spent two hours reading
• but I completed only half

In Polish, tylko usually goes right before the part it emphasizes.

Yes. Polish word order is more flexible than English, because case endings already show the grammatical roles.

For example, these are all possible with different emphasis:

• Wczoraj czytałam raport przez dwie godziny, ale przeczytałam tylko połowę.
• Czytałam wczoraj raport przez dwie godziny, ale przeczytałam tylko połowę.
• Raport czytałam wczoraj przez dwie godziny, ale przeczytałam tylko połowę.

The original version is very natural and neutral. Putting wczoraj first helps set the time frame immediately.

No. It is often placed near the beginning, but it can appear elsewhere.

Examples:

• Wczoraj czytałam raport.
• Czytałam wczoraj raport.
• Raport czytałam wczoraj.

These all mean roughly the same thing, but the emphasis changes slightly. Beginning with wczoraj is very common when you want to establish when something happened first.

Because ale means but, and in Polish it introduces a new clause. A comma is normally used before conjunctions like ale.

So:

• czytałam raport przez dwie godziny, ale przeczytałam tylko połowę

This is standard punctuation.

In English, yes, that can work depending on context. But grammatically, the Polish czytałam is clearly imperfective, so it is closer to:

• I was reading the report for two hours
• I spent two hours reading the report

That is why the second part is important: it shows the action was not completed, since the speaker read only half.

Normally, przeczytałam is a perfective verb and suggests a completed reading of whatever object it takes. But here the object is połowę.

So the completed action is not finishing the report, but finishing half of it.

In other words:

• przeczytałam raport = I finished reading the report
• przeczytałam połowę raportu = I finished reading half of the report

The perfective verb marks completion of the object actually stated.

Because the first clause is about duration of the activity, not about its result.

• czytałam raport przez dwie godziny = what I was doing, and for how long
• ale przeczytałam tylko połowę = what result I achieved

This kind of sentence is very typical in Polish: first the process, then the actual completed amount or result.