Subscripts count atoms
A subscript is the number of atoms of the element immediately before it. In SiO₂ there is one silicon and two oxygen. Where no subscript appears, the count is one.
This is why quartz is written SiO₂ and not Si₁O₂. The convention is old, universal, and saves a great deal of ink.
Brackets group and multiply
A bracketed group is multiplied by whatever subscript follows it. In Ca₃Sc₂(SiO₄)₃ the SiO₄ unit appears three times, giving three silicon and twelve oxygen from that group alone.
Bracketed groups usually correspond to something structurally real — a silicate tetrahedron, a carbonate triangle, a sulfate group — rather than being a bookkeeping convenience.
The dot means water, loosely attached
A centred dot introduces water molecules held in the structure but not bonded the way the rest is. Gypsum is CaSO₄·2H₂O: calcium sulfate with two waters per formula unit.
Those waters are why gypsum dehydrates to bassanite and then anhydrite when heated. Anything after a dot is the first thing to leave.
Superscripts are charge, not count
Fe²⁺ and Fe³⁺ are the same element in different oxidation states, and minerals routinely contain both. Magnetite is written Fe²⁺Fe³⁺₂O₄ for exactly that reason.
Confusing a charge for a count is the most common beginner error. If the number is raised and followed by a plus or minus, it is charge.
Commas mean the composition varies
A group such as (Ca,Na) means those sites can be occupied by either element, in proportions that vary between specimens. There is no single correct composition for such a mineral, and no single molar mass.
This is real chemistry rather than sloppy notation. Solid solution series are continuous, and a formula with commas is telling you honestly that a single number would be a lie.
Practice on three
Halite is NaCl: one sodium, one chlorine, no brackets, no water. Gypsum is CaSO₄·2H₂O: a calcium, a sulfate group, and two waters loosely held. Magnetite is Fe²⁺Fe³⁺₂O₄: iron in two oxidation states, one of the first and two of the second, with four oxygen.
Those three between them use every convention above. A formula that resists reading after that is usually resisting because it contains a substitution group, which is the one case where there is no single right answer to find.