aldehyde
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An aldehyde is a carbonyl group (C=O) that sits at the very end of a carbon chain, with a hydrogen atom directly attached to the carbonyl carbon. The general shape is R-CHO, where R is the rest of the molecule (or just another hydrogen, as in formaldehyde). The familiar smell of almonds (benzaldehyde) and the sting of formaldehyde both come from this family.
Naming follows the parent alkane: count the longest chain that includes the C=O carbon, drop the final -e, and add -al. So a two-carbon aldehyde is ethanal (common name acetaldehyde), a three-carbon one is propanal. The aldehyde carbon is always carbon number 1 because it must be at the end of the chain. In a condensed formula it is written -CHO (not -COH, which would wrongly suggest an O-H bond); the H is bonded to carbon, the O is double-bonded.
Aldehydes matter because they are unusually reactive carbonyls and are central building blocks in synthesis. The lone hydrogen on the carbonyl carbon is also why aldehydes can be oxidized easily (to carboxylic acids) while ketones cannot, a difference exploited in classic tests like Tollens' silver mirror.
CH3CHO is ethanal (acetaldehyde), the two-carbon aldehyde formed when ethanol is partially oxidized; it is the compound your liver makes from alcohol and a cause of hangovers.
Ethanal: a carbonyl at the chain end with one H on the carbonyl carbon.
Write the group as -CHO, never -COH: the latter looks like an O-H (alcohol) bond, which is wrong. The H is on carbon and the O is doubly bonded.