[renbri]

I thought, in a chemical formula, a regular number indicated the number of molecules of the element that followed (e.g. 2C would be two molecules of carbon) and a subscript indicated the number of atoms in the molecule of the preceding element (e.g. H₂ would be a hydrogen molecule with two atoms -as normal).
That would mean that H₂O signified a two-atom molecule of hydrogen bonded to a molecule of oxygen -which seems okay. But I often see it written H2O and to me that ought to mean one molecule (or atom?) of hydrogen and two of oxygen!
Evidently I don't understand the rules properly, Could somebody please tell me where I'm going wrong? Thanks.

[Arkcon]

Briefly, H2O means that someone has forgotten to write the subscript.  That's pretty much it.

[Borek]

Briefly, H2O means that someone has forgotten to write the subscript.  That's pretty much it.

Or was too lazy to format it properly.

Evidently I don't understand the rules properly, Could somebody please tell me where I'm going wrong? Thanks.

It is not a problem with your understanding.

Note that we never use regular numbers inside of the formula without preceding them by a middot, like in CuSO₄·5H₂O or CaO·Al₂O₃·2SiO₂.

[DrCMS]

e.g. 2C would be two molecules of carbon

No that is 2 atoms of carbon

H₂ would be a hydrogen molecule with two atoms -as normal).

Yes

That would mean that H₂O signified a two-atom molecule of hydrogen bonded to a molecule of oxygen -which seems okay.

No it is 2 atoms of hydrogen bonded individually to the oxygen.  There is no H-H bond in water.  There are two H-O bonds, see the molecule below.