“Corn”is an uncountable noun . Can it be used in a sentence with other countable nouns? Yes. I am confused with lettuce ,if it is a countable noun or not. Countable if you want to count lettuces. For example, buy three lettuces. Uncountable if you want to say how much chopped lettuce to put in a sandwich. Just give me lots; I like lettuce. . Buying at the supermarket: I want cucumbers, corn and lettuces. Ordering a sandwich filling at Subway: want cucumber, corn and lettuce. . Mixed in asentence. No problem.

Lettuce, at least in the U.S., is not countable. You can mix countable and non-countable nouns in the same sentence. Usually, in a sentence where we actually count one noun, we use a partitive expression with the non-countable nouns. It just sounds more balanced that way, but it's not obligatory. For example: 1. I want cucumbers, corn, and lettuce. 2. I want 4 cucumbers, some corn and some lettuce. 3. I want 4 cucumbers, 2 ears of corn, and 3 heads of lettuce. 4. I want corn, lettuce, and 4 cucumbers.