[Minerva]

If you react acetaldehyde with methanol in acidic conditions, I assume hemiacetal is unstable and proceeds to form acetal right?  But why does my chemistry textbook say the final product is hemiacetal?

[orgopete]

In this case, the description is insufficient to accurately predict what the products might be. Because aldehydes are both quite reactive and easily oxidized, simply mixing methanol and acetaldehyde would produce some hemiacetal. It would likely contain some acetic acid, so you could consider it acid catalyzed. The book may have wished to use "acid catalyzed" to match the conditions of acetal formation.

Since this reaction is an equilibrium reaction, removal of water during the reaction favors acetal formation. Addition of water can drive an acid catalyzed hydrolysis toward the aldehyde. Therefore, the description probably would not clearly delineate exactly what is happening. A better clue to predicting the product is the context of the question. If it is about hemiacetal formation, then these conditions, incomplete as they are, are probably sufficient to conclude hemiacetal formation.

N.B., For a reaction like this, an orthoester is used in an exchange reaction to absorb water formed and drive the equilibrium toward an acetal.