American T: How Americans Say the "T" Sound in the Start, Middle and Ends of Words

In American English, we have 3 different ways of making a "T" sound.  For all of the /t/ sounds, we use the tip of the tongue and the alveolar ridge (the roof of the mouth just behind the teeth). In the start of words, we always use an aspirated t.  This means we have a puff of air which escapes when we release the t.

At the ends of words, Americans often use a stopped T.  This means they move the tongue into position for the /t/, but they don't release the air.

In the middle of words, we use aspirated T, stopped T, and flapped T.  A Flap T is different because the voice is on (unlike the other types of T, which are voiceless), and a flap T does not have any build-up of air like a T or a D.  The tongue contacts the alveolar ridge lightly and releases immediately.  If you have difficulty with flap T, it can help to think of it as a light D sound.

Unfortunately, we don't always use a flap T in the middle of words.  The video below gives you examples of the following patterns:

Medial T is usually flapped when between 2 vowel sounds. This includes final T when the next word begins with a vowel and we link words. (Final T Linking is discussed here.)
Most words with -ter, -ted, -test, -ting, -ty
better, batter, butter, letter, waiter, writer, water, litter, greater, lighter, wetter
waited, righted, lighted, matted, sighted, fitted,
metabolic,  artificial, comforting, dirty, city
greatest, hottest, fatal, hospital, turtle

Medial T (stopped when followed by syllabic n or -ment, -ance, -ly, -m)  
Button     kitten    mittens        rotten        cotton    mountain   threaten
Appointment, department, commitment, disappointment, adjustment, treatment
Important, Importance, combatant, pollutant,
Ultimately, infinitely,
Atmosphere

Medial T between 2 vowels (aspirated when second syllable is stressed, other exceptions)
Guitar  potato    eighteen   hotel  motel  retirement   material
Architecture   laboratory   irritant   militant

Medial T (aspirated when not any of the above:  in a cluster, not between two vowels)
Distant, Opportunity, transportation, Intonation, enter, interpretation, reputation, representation, concentrate, multi-, sophisticated, documentation, investigate, particular, metro-, consultation, continuation, anticipation

Medial nt words, can sometimes omit t:  wanted, Santa Barbara, counter, anti- (eg antibiotic)  inter-(eg international),  representative, identification, disappointing,  advantage