Before you can talk about reactions you have to be able to read and draw molecules fast, and organic chemists have developed shorthand precisely because drawing every atom in a large molecule is unbearably slow. There are three levels of representation, and true fluency means being able to translate any structure between all three without thinking about it.

The most detailed is the full Lewis (expanded) structure, where every atom and every bond is drawn explicitly, including every C–H bond and every lone pair. This is honest and complete but painfully cluttered — imagine drawing all 20 hydrogens on an eight-carbon molecule. You use it only when you specifically need to track every electron, such as when first learning a mechanism.

The middle level is the condensed formula, such as CH₃CH₂CH₂OH for propanol. Here the bond lines mostly disappear and hydrogens are written immediately after the carbon they belong to. You read it left to right as a chain: CH₃ (a carbon with three H's) bonded to CH₂ (a carbon with two H's) bonded to another CH₂ bonded to an OH. Parentheses show branching: (CH₃) hanging off a chain means a methyl branch at that point. Condensed formulas are compact for simple molecules but get confusing for anything branched or cyclic.

The working language of the entire course, though, is the skeletal structure, also called the line-angle structure. It is built on a few conventions that feel strange for about a day and then become second nature:

Every line is a bond, and every vertex (bend) and every line end (terminus) represents a carbon atom. You do not write the letter C — the corners are the carbons.
Hydrogens attached to carbon are invisible. You never draw them. Instead, you mentally supply however many H's are needed to bring each carbon up to its required four bonds. A carbon at the end of a line with only one bond shown silently carries three hydrogens; a carbon at a bend with two bonds shown silently carries two; and so on.
Heteroatoms — any atom that is not carbon or hydrogen (O, N, S, halogens) — are always written out explicitly with their letter symbol. And, importantly, hydrogens attached to a heteroatom must be drawn (the O–H of an alcohol, the N–H of an amine). The reason is not arbitrary: these heteroatom hydrogens are the chemically interesting, often acidic or hydrogen-bonding ones, so hiding them would hide the reactivity.
Double bonds are two parallel lines, triple bonds are three.

Why does this matter beyond neatness? Because the invisible hydrogens are not really invisible to the chemistry — miscounting them changes the molecular formula, the formal charge, and the reactivity you assign. If you glance at a ring carbon and forget it silently carries a hydrogen, you may wrongly think it has a lone pair or a charge. So a good exercise, especially early, is to take any skeletal structure and explicitly pencil in the hidden hydrogens on a few carbons to confirm each one has exactly four bonds. Do this until you trust the shorthand, then let the hydrogens go invisible again.

A related skill is recognizing the degree of unsaturation (also called index of hydrogen deficiency) from a structure or formula — essentially counting how many rings and pi bonds a molecule has. Each ring and each pi bond removes two hydrogens compared to the fully saturated chain. You will use this constantly later to interpret spectra, so start noticing it now: a molecule with one ring and one double bond has two "degrees of unsaturation."

"Line 'Em Up"