English pronounciation question, please help.?

In conversational English in the U.S.:
1. The letter T is usually aspirated at the beginning of a stressed syllable, or when after another consonant sound (except R), or when followed by an R sound. Examples: what's, terminal, succinct, angioplasty, banter, nitro, deter
2. In your examples, the T does not come at the beginning of a stressed syllable. It also comes after a vowel sound, and followed by a voiced consonant sound or by nothing (end of the word/group of words). This T is not silent; your tongue closes the airflow by quickly hitting the roof of the mouth, just behind your top teeth. These are unvoiced like a regular T, but are not plosive, not aspirated. These are called glottal stops. Examples: what, "that was," immediately, Kathmandu.
3. On the other hand, when a T (especially double T) is not at the beginning of the stressed syllable, and appears between two vowel sounds (this includes the sound R before the T, or the sound L after the T), it is usually voiced identical to the letter D. Most people do not realize they pronounce it this way, but they do. Examples: "what is," pottery, letter, it'll, forty

None of these are silent, but the first one is definitely the least silent.
Because this is English there are many exceptions, and there are many variations (some people might aspirate both Ts is the word "student," some people might pronounce "letter" more like a glottal stop). The main exception to these pronunciations is when the speaker wants to give emphasis to the word, like when someone asks for something to be repeated. In that case, every T could be fully aspirated. And note that fully aspirating the T is never a "mistake," but if you do aspirate everything, people will think your speech is very formal and impersonal.

the Ts in these situations are pronounced as glottal stops
http://www.youtube.com/watch?v=edxwQK1zBxw

You pronounce the "T" like any other consonant. Clearly