There are several atom combinations that show up again and again in molecules and affect the chemistry of the molecules. These functional groups are part of the "language" of biological chemistry, and it can be very useful to have most of them committed to memory.
All of these groups tend to be polar, and it is often the polarity characteristics that help determine their chemistry:
ALDEHYDE, -C=O on an end carbon.
KETONE, -C=O on an internal carbon.
ACETYL GROUP, end grouping with a ketone just inside an end methyl group (-CH3).
METHYL GROUP, -CH3 that can be added or removed to activate large molecules.
CARBOXYLS, ACID GROUPS, -COOH. These will generally give up the hydrogen on the end as an ion, adding such H+ ions to solution, which is why it's an acid.
AMINES / AMINO GROUPS, -NH2. These tend to pick up hydrogen ions from solution, making the solution basic.
THIOLS (MERCAPTANS), -SH. When found inside large molecules, loses hydrogens and covalently bonds to other sulfurs, forming covalent internal bridge-like connections that can help to hold a molecule in a stable form.
PHOSPHATES, -POX. Varies in form according to pH. Commonly moved onto and off molecules as an energy carrier.
Again, this terminology is used so commonly that it should be part of your basic vocabulary.