If the festival wasn't so expensive, we would celebrate it. (Instead of directly stating the reason, we use "if" to express a hypothetical situation where the reason doesn't exist.)

Most people cannot return home for Tet if they haven't booked their tickets long in advance. (Reversing the cause-and-effect relationship with "if" emphasizes the consequence of not booking early.)

Although Hung speaks English well, Nam is not far behind. (This avoids using "badly" which has a negative connotation, and instead highlights Nam's proficiency through comparison.)