Where subscripts are used
A subscript sits below the baseline. In chemistry it counts atoms: H₂O has two hydrogen atoms and one oxygen atom, and CO₂ has two oxygens. In math and science it labels things: x₁ and x₂ are two different values, v₀ is a starting velocity, and log₂ is a logarithm in base 2.
Ions use both directions at once, with the atom count below and the charge above: SO₄²⁻. Copy the raised part from superscript symbols.
Characters vs formatting
The subscripts here are Unicode characters (mostly U+2080 to U+209C), not formatting. Paste H₂O into a chat, a spreadsheet cell, a product title or a social bio and the 2 stays low. Subscript formatting from Word or Google Docs is lost as soon as the text lands somewhere plain.
The catch is coverage. All ten digits and the signs + − = ( ) exist, but only part of the alphabet has a subscript letter, so you cannot write every word this way. Letters such as ₕ ₖ ₗ ₘ ₙ ₚ ₛ ₜ were added much later than the digits and may show as empty boxes on older devices. For whole words, formatting is the safer choice.
How to type subscript
In Word, select text and press Ctrl + = for subscript formatting. To insert the character itself, type its code, such as 2082 for ₂, and press Alt + X. In Google Docs, press Ctrl + , (Command + , on a Mac). In LaTeX, write x_1 in math mode, or \ce{H2O} with the mhchem package for chemistry.
Windows has no Alt codes for subscript digits, and a Mac has no built-in shortcut for them, so the fastest route on computers and phones alike is to copy them from this page or save formulas you use often, like H₂O, as text replacements. For numbered lists, try the number symbols instead.