in dealing with alcohol mechinisms, how do you know when to do dyhydration (elimination) or substitution? is it secondary and teritary undergo both Sn1 & E1 and primary undergo Sn2 and E2. or is one mech actually preferred? it seems like H2SO4 is always dehydration.

Well, for alcohols they in general prefer E1 elimination for tertiary and secondary alcohols. For primary alcohols however they do E2 because the activation energy is so high that the water will not just "fall" off on it's own. The key is to use high temperatures for alcohols to get elimination.

H2SO4 is not a alcohol by the way, but rather a acid.

It's used for dehydrations of alchohols but it's different for primary, secondary, and tertiary alchohols (strength wise and temperature wise as well).

Primary alchohols: concentrated H2SO4
temperature: 180 deg Celcius

Secondary alchohols: 85% H3PO4
temperature: 170-165 deg Celcius

Tertiary alchohols: 20% H2SO4
temperature: 85 deg Celcius