Learn Hardcore German: Ich trinke Wasser beim Lesen.

Beim is the contraction of bei dem. With activities, bei often means during/while. So beim Lesen literally means bei dem Lesen (at/during the reading) and idiomatically means while reading.

Why is Lesen capitalized?

Because it is a verb used as a noun (a nominalized infinitive). In German, all nouns are capitalized. Here lesen (to read) becomes the noun das Lesen (the act of reading), required after bei(m).

What case is Lesen in, and why?

Dative singular. The preposition bei always takes the dative. Since the noun is neuter (das Lesen), bei dem Lesen contracts to beim Lesen.

Where is the article for Lesen? Why don’t I see it?

It is inside the contraction: bei dem → beim. Uncontracted, the sentence would be Ich trinke Wasser bei dem Lesen, which is grammatical but sounds stiff; native speakers strongly prefer the contraction beim Lesen.

Why is there no article before Wasser?

Wasser is an uncountable mass noun and, when used in a general sense, German normally drops the article: Ich trinke Wasser. Use an article for specificity or countable servings:

Ich trinke das Wasser (that specific water)
Ich trinke ein Wasser (a serving, e.g., a glass/bottle, common when ordering)

Is Ich trinke ein Wasser correct and what does it imply?

Yes. Ich trinke ein Wasser is idiomatic in contexts like cafés and restaurants and usually means one serving (glass/bottle) of water, not water in general.

Can I put beim Lesen somewhere else in the sentence?

Yes. Common options:

Ich trinke Wasser beim Lesen.
Ich trinke beim Lesen Wasser.
Beim Lesen trinke ich Wasser. All are correct. Fronting beim Lesen emphasizes the reading context; putting it at the end is neutral-to-end-focus; placing it after the verb often sounds slightly more flowing in speech. The finite verb must remain in second position.

Why is the verb trinke in second position?

German main clauses follow the V2 rule: the finite verb occupies the second position. In Ich trinke Wasser beim Lesen, the first element is Ich, so trinke is second. If you front the time phrase, you still keep V2: Beim Lesen trinke ich Wasser.

Do I need a comma in the original sentence?

No. Ich trinke Wasser beim Lesen. has no clause boundary, so no comma. If you use a subordinate clause, you do need one: Ich trinke Wasser, während ich lese.

What is the difference between beim Lesen and während ich lese?

Both express simultaneity.

beim Lesen = prepositional phrase with a nominalized verb; compact and idiomatic for activities.
während ich lese = subordinate clause; slightly more explicit and requires verb-final word order and a comma. A very formal written alternative is während des Lesens (genitive: des Lesens).

What exactly is the grammar of Lesen here? Does it decline?

It’s a nominalized infinitive, neuter: das Lesen. Declension:

Nominative/Accusative: das Lesen
Dative: dem Lesen (→ beim Lesen)
Genitive: des Lesens (e.g., während des Lesens) With adjectives: beim konzentrierten Lesen (dative singular).

Does bei always take the dative? More examples?

Yes. Bei is a dative preposition. Examples:

bei der Arbeit (at work)
bei den Hausaufgaben (while doing homework / with the homework)
bei mir zu Hause (at my place)

Is am Lesen the same as beim Lesen?

No. am is a contraction of an dem and is used in some regions for a progressive with sein: Ich bin am Lesen (I’m reading right now). You cannot use it to modify another verb the way beim does. So say Ich trinke Wasser beim Lesen, not Ich trinke Wasser am Lesen.

How is trinke formed, and does it mean I am drinking right now or I drink in general?

trinke is the first-person singular present of trinken. German’s present covers both current and habitual actions. Context decides:

Right now: add gerade if you want to stress it: Ich trinke gerade Wasser beim Lesen.
Habitual: Ich trinke Wasser beim Lesen can also mean you typically do this when you read.

What is the gender of Wasser, and how do I refer back to it?

Wasser is neuter (das Wasser). Use es as the pronoun: Das Wasser ist kalt. Es schmeckt gut.

Is there a plural of Wasser?

Rarely, Wässer exists to talk about types of water, especially mineral/medicinal waters: verschiedene Heilwässer. For everyday drinking water, you use the singular Wasser.