Learn Hardcore German: Wasser tropft vom Dach.

In German all nouns—no matter where they appear in a sentence—are written with a capital first letter. Since Wasser is a noun, it must be capitalized.

Why is there no article before Wasser?

Wasser is an uncountable mass noun in German. When you talk about water in general (not “a glass of water” or “the water we drank”), you normally omit the article. If you wanted to refer to specific water, you could say das Wasser, but for a generic statement you leave out the article.

What does vom mean, and why not just von or dem separately?

vom is simply the contraction of von + dem.

von is a preposition meaning “from.”
dem is the dative form of the neuter article das.
In everyday speech and writing you always contract von dem → vom.

Which case is used in vom Dach, and why?

The phrase vom Dach is in the dative case. In German, the preposition von always takes the dative, so dem Dach becomes vom Dach. Here it functions as an adverbial of origin/place (“from the roof”).

How do I know that Dach is a neuter noun?

You learn each German noun with its article. Dach is always das Dach in the dictionary, so it’s neuter. In the dative singular, das → dem, giving vom Dach.

Why does the verb tropft appear in the second position?

German main clauses follow the “verb second” (V2) rule: the finite (conjugated) verb must be the second element in the sentence. Here your first element is the subject Wasser, so the verb tropft comes right after it.

Why does tropft end in -t instead of -en or -et?

This is the 3rd person singular present tense ending for regular (weak) verbs in German: stem + -t.

The infinitive is tropfen (stem: tropf-).
3 SG present is tropf
- t = tropft.
  You only add an extra -e- before endings (→ -et) if the stem already ends in -t or -d (e.g. arbeiten → arbeitet).

Can I say Es tropft Wasser vom Dach instead? What’s the difference?

Yes, you can. In that version:

Es is a dummy subject, serving no meaning except to fill the subject slot.
You then introduce Wasser as the real subject/object of the verb.
Stylistically, Wasser tropft vom Dach (subject first) is more direct. Es tropft Wasser vom Dach puts focus on the fact that “something is dripping,” and then specifies that it’s water.

How would I express the past tense (“water dripped from the roof”) in German?

You have two main options:
1) Simple past (Präteritum) – more common in writing:
Wasser tropfte vom Dach.
2) Present perfect (Perfekt) – common in speech:
Wasser hat vom Dach getropft.