Hi Johnleo,
I guess you are learning American English, aren't you? In American English...
...I think the t in network is a glottal stop, like in "not". You move your tongue as if you were going to pronounce the t, but when the tongue is in the right position and ready for a t you don't pronounce it. It's difficult to explain, maybe someone else can explain that better. So in t+w the t is a glottal stop.
When there's a t+l the t is tapped, that is, it becomes a slight d. So "bottle" almost sounds like "bodle".
And when there's a t+n... well, it's difficult to explain. Either you pronounce both the t and the n as "t-uh-n", or you pronounce them as a single sound, not releasing the tongue but keeping it in position for both the t and the n.
As for t+s, I think the result is a "ts" sound, but I don't know what to say about that. The t is pronounced anyway.
Sorry if I can't be of much help, hopefully someone else will tell you more and correct me if I was wrong.