Comment by PeterStuer

The syntax is the set of rules that define what are valid expressions in the language. Valid as in the token sequence you wrote does not violate the grammar, nothing more that that. It can state that an "if" token must be followed by a "condition clause" that must be followed by "then" token etc.

The semantics is the definition of what is supposed to computationally happen if you execute a valid expression. It would state that the code block under the "then" will be executed if the condition attached to the "if" evaluated to true and skipped otherwise.