Learn Hardcore German: Der See ist tief.

Because See meaning a lake is masculine in German, so the nominative singular definite article is der. There is also die See (feminine) meaning the sea in maritime/literary German, but in everyday use the sea is das Meer.

Why is ist in the second position?

German main clauses follow the verb‑second rule. One element (here, the subject Der See) comes first, the conjugated verb ist comes second, and the rest follows. If you move another element to the front, the verb still stays second: Heute ist der See tief.

Why doesn’t tief have an ending like tiefe?

Because tief is used predicatively after sein; predicative adjectives are not inflected. Before a noun (attributive), it takes an ending: der tiefe See, ein tiefer See, tiefe Seen.

How do I turn this into a question?

Yes–no question: Ist der See tief?
With a wh-word: Wie tief ist der See?

How do I negate it?

Place nicht before the adjective: Der See ist nicht tief. You can also use an antonym: Der See ist flach.

How do I pronounce the words?

Der: [deːɐ̯]
See: [zeː] (initial s before a vowel sounds like English z; ee = long [eː])
tief: [tiːf] (ie = long [iː])

What’s the plural of See (lake)?

die Seen. Example: Die Seen sind tief.

Which case is used here, and how does See decline?

Nominative, because Der See is the subject. Declension:

Nominative: der See
Accusative: den See
Dative: dem See (e.g., am See = an dem See)
Genitive: des Sees (e.g., die Tiefe des Sees)

What’s the difference between am See and im See?

am See = at the lake, on the lakeshore (an + dem)
im See = in the lake (in + dem), i.e., in the water

Is there a difference between der See, die See, and das Meer?

der See = a lake (masculine)
die See = the sea (feminine; more maritime/literary; in names like Nordsee, Ostsee)
das Meer = the usual everyday word for the sea (neuter)

Can I replace Der See with a pronoun?

Yes. Since See is masculine, use er in the nominative: Er ist tief. In other cases: Ich sehe ihn, mit ihm.

How do I add intensity (very/quite/really)?

Common adverbs:

sehr tief (very)
ziemlich tief (quite/fairly)
wirklich tief (really)
nicht besonders tief (not particularly)

What are the comparative and superlative forms of tief?

Comparative: tiefer — Der See ist tiefer als der Fluss.
Superlative (predicative): am tiefsten — Dieser See ist am tiefsten.
Superlative (attributive): der tiefste See

Does word order change if I start with another element?

No. The verb still stays second:

Vielleicht ist der See tief.
Im Sommer ist der See tief. (Grammar is fine; the meaning may be odd—depth doesn’t usually change daily.)

Anything to watch out for with capitalization?

Yes. Nouns are capitalized (See), adjectives are not (tief), and the first word of the sentence is capitalized (Der).

Is See related to the verb sehen?

They look similar but are different words. See (lake/sea) is a noun; sehen is a verb meaning to see. Pronunciation differs: See [zeː], sehe [ˈzeːə].