In casual American English, "I don't know what to tell you" sounds like "ahy dohn NOH wuht tuh TEHL yoo". Three things happen here, and the headline one is the Same-Consonant Linking: the doubled consonant is held once, not pronounced twice. Keep stressed words long, unstressed words short, and link the consonants forward into the vowels.
Now you try.
Read the sentence out loud at native speed. The mic stays on your device — nothing's uploaded.
What makes this sentence sound American.
The "" shared between "don't" and "know" is held once, slightly longer, and released once instead of stopping and starting twice. This is called the Same-Consonant Linking, what turns word-by-word reading into actual conversation. It comes out as dohn.
What's happening in this sentence.
Small tricks that turn a textbook sentence into how an American actually says it.
Common pronunciation mistakes in American English.
The textbook way isn't wrong — it's just not how anyone actually says it.
Treating every L the same.
The L in "tell" is a dark L — the back of the tongue rises toward the soft palate, adding a small "uh" quality before the L. Dark L adds a small schwa-like "uh" before the L. The back of the tongue lifts toward the soft palate.
Pronouncing the identical consonant twice.
The "" shared between "" and "" is held once, slightly longer, and released once instead of stopping and starting twice. Consonant is held slightly longer and released once (not said twice).
Pronouncing the function word too fully.
"to" is a function word — in connected speech, the full vowel reduces to a quick "" sound and consonants may simplify. Full vowel reduces to schwa /ə/ or other weak vowel. Consonants may simplify.