For addition, you use the + symbol e.g. a + b = c: $$ a + b = c$$

For subtraction, you use the - symbol e.g. a - b = c $$ a - b = c$$

For multiplication, you just write ab or put a dot in between with \cdot (meaning: “centered dot”) like a \cdot b.
Using a * b doesn't look natural in most cases $a * b$ (but it might make sense sometimes!) $$a\cdot b = c$$

For fractions, you use \frac{a}{b} or {a \over b}.
You can use a / b if you don't want to make a fraction $a / b$. $${a \over b} = c$$

For modulo, you can use a \% b if it's. The percent symbol is comment % ... comment ....
Usually in mathematics, you write modulo as “congruence modulo N” which you do like a = b \pmod{N} $$ a = b \pmod{N}$$

For exponentiation, you use e^{ ... } e.g. e^{i\phi} = \cos(\phi) + i\sin(\phi).
When it's a single variable in the exponent, omitting braces is fine e.g. e^\phi or e^{\phi} are both fine. $$e^{i\phi} = \cos(\phi) + i\sin(\phi)$$

For logarithm, you use \ln. $$\ln(1 + x) = 1 + x + $$

For integrals, you use \int, \iint, \iiint, \oint, \oiint, \oiiint. $$\int \iint \iiint \oint \oiint \oiiint$$