In der Übung sollen wir zwei Sätze ankreuzen und drei Wörter unterstreichen.

Because in can take either the dative or the accusative, depending on meaning.

• dative = location, being in something
• accusative = movement into something

Here, in der Übung means in the exercise or in this exercise, so it describes location/context, not motion. That is why German uses the dative:

• die Übung → feminine noun
• dative feminine → der Übung

So:

• in der Übung = in the exercise
• in die Übung would suggest movement into the exercise, which does not fit here

Here sollen means something like are supposed to, should, or are meant to.

So sollen wir ... ankreuzen und ... unterstreichen means:

• we are supposed to tick two sentences and underline three words

This use is common in instructions, classroom language, and tasks.

It is slightly different from:

• müssen = must, have to
• können = can, be able to
• wollen = want to

So sollen often suggests an instruction, expectation, or assignment rather than pure necessity.

Because the sentence begins with In der Übung, which takes the first position in the sentence. In German main clauses, the finite verb must still come in second position.

So the structure is:

• In der Übung = position 1
• sollen = position 2
• wir = comes after the verb

That gives:

• In der Übung sollen wir ...

If the sentence started with wir, then you would get:

• Wir sollen in der Übung ...

Both are grammatically correct, but the original sentence puts emphasis on the context in the exercise.

Because sollen is a modal verb, and in a main clause with a modal verb, the other verb or verbs usually appear in the infinitive at the end.

Here:

• finite verb: sollen
• infinitives: ankreuzen, unterstreichen

So the pattern is:

• subject + modal verb + other information + infinitive(s)

Example:

• Wir sollen lesen.
• Wir sollen zwei Sätze ankreuzen.
• Wir sollen zwei Sätze ankreuzen und drei Wörter unterstreichen.

Both main actions stay in the infinitive because they depend on sollen.

Yes. ankreuzen is a separable verb.

Its parts are:

• an- = separable prefix
• kreuzen = base verb

In a normal main clause without a modal verb, it separates:

• Wir kreuzen zwei Sätze an.

But with a modal verb, it stays together in the infinitive:

• Wir sollen zwei Sätze ankreuzen.

That is why you see ankreuzen as one word at the end here.

Because after a number greater than one, German normally uses the plural.

The singular is:

• der Satz = the sentence

The plural is:

• die Sätze = the sentences

So:

• zwei Sätze = two sentences

Notice that the plural has both:

• an umlaut: a → ä
• the ending -e

This is just the correct plural form of Satz.

Both Wörter and Worte exist, but they are used differently.

• Wörter usually means individual words as vocabulary items
• Worte often refers to words in a more connected or expressive sense, like remarks, speech, or famous words

In an exercise, where you count separate word items, Wörter is the natural choice:

• drei Wörter unterstreichen = underline three words

So this sentence uses the standard form for countable vocabulary items.

Because numbers often directly modify nouns without an article.

So German commonly says:

• zwei Sätze
• drei Wörter
• fünf Bücher

This works like English:

• two sentences
• three words

You could add other determiners in some contexts, for example:

• die zwei Sätze = the two sentences
• diese drei Wörter = these three words

But here the plain number is enough.

They are in the accusative case, because they are the direct objects of the verbs:

• ankreuzen what? → zwei Sätze
• unterstreichen what? → drei Wörter

In this sentence, the accusative forms look the same as the nominative plural:

• die Sätze / zwei Sätze
• die Wörter / drei Wörter

So even though they are accusative, their form does not change here.

Because German usually does not use a comma before und when it simply joins two equal parts of a sentence.

Here, und links two infinitive phrases:

• zwei Sätze ankreuzen
• drei Wörter unterstreichen

So no comma is needed:

• ... zwei Sätze ankreuzen und drei Wörter unterstreichen.

A comma might appear in more complicated structures, but not in this basic coordination.

In this sentence, Übung most naturally means exercise in the sense of a task or activity, especially in a schoolbook, worksheet, or language course.

So In der Übung usually means:

• In the exercise
• In this exercise

The word Übung can also mean practice in other contexts, but here the rest of the sentence clearly sounds like workbook instructions, so exercise is the best fit.

Yes, absolutely. That is also correct.

German word order is flexible as long as the finite verb stays in second position in a main clause. So both of these are fine:

• In der Übung sollen wir zwei Sätze ankreuzen und drei Wörter unterstreichen.
• Wir sollen in der Übung zwei Sätze ankreuzen und drei Wörter unterstreichen.

The difference is mainly emphasis:

• In der Übung ... highlights the exercise itself
• Wir sollen ... starts with we, so it focuses more on the people doing the task