Learn Dutch: Wij kruipen onder de zware boomstam door. - We crawl under the heavy tree trunk.

It is part of the circumposition (a two-part preposition) onder ... door. While onder tells us the action happens "under" the trunk, adding door at the end indicates movement or a path. It shows they are passing under the trunk from one side to the other.

Would the sentence still be correct if I left out the word door?

It would be grammatically correct, but the meaning changes. Wij kruipen onder de zware boomstam implies you are crawling around in a fixed location underneath the trunk. The added door clarifies that you are moving through the space underneath it.

Why is the adjective written as zware instead of zwaar?

Because boomstam is a de word, the adjective in front of it gets an -e ending. When you add the -e to zwaar, the syllable opens up (zwa-re). By Dutch spelling rules, we drop the second 'a' because a single vowel in an open syllable already produces a long sound.

How do we know the gender of a compound word like boomstam?