Fay hattı tehlikeli.

Turkish simply doesn’t have definite or indefinite articles like English does. You state the noun on its own and context tells you if it’s “a” or “the.”

• Fay hattı tehlikeli. can mean “The fault line is dangerous.”
• If you really want to say “A fault line is dangerous,” you could add bir: Bir fay hattı tehlikeli.

No, it’s not the direct-object (accusative) marker here. It’s the 3rd person singular possessive suffix.

• hatt (line) + -ı → hattı literally “its line.”
• fay hattı therefore literally means “the fault’s line” → English “fault line.”

The accusative suffix in Turkish also looks like -ı/-i/-u/-ü, but it usually attaches to the end of a phrase and often has a buffer -y after a vowel: e.g. kitabı (“the book” as object). Here there’s no buffer y, and the context (possession in a compound) is different.

In Turkish the present-tense copula (to be) for 3rd person is usually omitted. You simply put Subject + Predicate.

• Fay hattı tehlikeli. = “The fault line is dangerous.”
  If you want to be very formal or emphasize it, you can add the suffix -dir to the adjective:
• Fay hattı tehlikelidir.

Turkish follows a Subject-Object-Verb (SOV) or more generally Nominal-Predicate order. In a simple “X is Y” sentence, the noun (X) comes first, the adjective or predicate (Y) comes last:

• [Subject] Fay hattı
  • [Predicate] tehlikeli.

Each Turkish letter is pronounced. Rough phonetic guide:

• Fay = /faj/ (“fie”)
• hattı = /ˈhat.tɯ/ (double t-sound; “hut-tuh” with a back, unrounded vowel for ı)
• tehlikeli = /teh.liˈke.li/ (“teh-lee-keh-lee”)

The Turkish ı (dotless i) is a high, back, unrounded vowel – somewhat like the “e” in English “taken,” but more centralized.