Wij lezen de vijftiende pagina.

Yes. We and wij both mean we.

• we is the common unstressed form in everyday Dutch.
• wij is the full form and is often used for extra emphasis or in a slightly more careful style.

So We lezen de vijftiende pagina is also correct and very natural.

Because the subject is wij. In the present tense, wij normally takes the same form as the infinitive.

For lezen:

• ik lees
• jij/u leest
• hij/zij leest
• wij lezen
• jullie lezen
• zij lezen

So wij lezen is the correct form.

It can mean both. The Dutch present tense often covers both ideas:

• we read
• we are reading

Context usually tells you which meaning is intended. If you want to make the ongoing action extra clear, Dutch can also say:

• Wij zijn de vijftiende pagina aan het lezen.

But the simple present Wij lezen... is completely normal.

Because pagina is a de-word. In Dutch, singular nouns take either de or het, and this is something you usually have to learn with each noun.

So:

• de pagina = the page

There is no reliable way to guess this from English, so it is best to memorize the noun together with its article.

Vijftiende is the ordinal form of vijftien.

• vijftien = fifteen
• vijftiende = fifteenth

Dutch ordinal numbers are often made with endings like -de or -ste. Here it is:

• vijftien
  • -de → vijftiende

So it works similarly to English fifteen → fifteenth.

Because ordinal numbers used before a noun behave like adjectives in Dutch. In that position, they normally appear with -e.

Examples:

• de vijftiende pagina
• de eerste dag
• het tweede hoofdstuk

So the -e is part of the normal form used before a noun.

This sentence has the basic Dutch main-clause order:

• Wij = subject
• lezen = finite verb
• de vijftiende pagina = object

So the pattern is subject + verb + object.

More generally, Dutch main clauses follow the verb-second rule. That means the finite verb comes second in the sentence:

• Wij lezen de vijftiende pagina.
• Vandaag lezen wij de vijftiende pagina.

In the second sentence, Vandaag comes first, so lezen still stays in second position.

A rough English-style approximation is VAYF-teen-duh, but that is only approximate.

A few points:

• ij does not have an exact English equivalent.
• vijf sounds roughly like vafe, but with a Dutch vowel.
• the final -de is usually a weak duh sound.

The best way to learn it is by listening to native audio, because English spelling cannot represent it very accurately.