When a coworker keeps sliding you their tasks, “I can’t take that on” is a real work skill.

Some coworkers hand off their work casually and count on you being too new or too polite to say anything. It starts with one small favor and quietly becomes the arrangement, because nothing ever went wrong for them when they tried it. Doing someone else’s job does not make you valuable. It makes you convenient.

The sentence you need is short and needs no reason attached: I cannot take that on right now. Say it in a normal, friendly voice and do not rush to fill the silence afterward. If it keeps happening, mention it once to whoever actually assigns your work.