In English, "前任" can be abbreviated as "ex" or "ex-." For example, you might refer to someone as your "ex-partner" if they were your former romantic partner.

Note that these abbreviations are often used informally and may not be appropriate in all situations. It is generally more common to simply use the full word "former" to refer to someone who used to hold a certain position or role. For example, you might say "my former boss" instead of "my ex-boss."