Great question. Both combine and merge mean to bring things together, but they are used a little differently. Combine is more general. It can mean putting two or more things together, but they can still stay separate in some way. For example, you can combine flour, sugar, and eggs when baking. Merge usually means two things come together and become one. For example, two companies can merge into one company, or two lanes on the road merge into one lane. So, combine is about mixing or joining, while merge is about becoming a single whole.

"Merge" connotes a gradual, smooth, and perhaps even gentle combination. "Combine" is more like "mix". If a scientist combines two chemicals, it just means she poured them together. It would be unusual to say that she "merged" two chemicals, but you could say it, and it would mean that she combined them slowly, deliberately, and with care. After you drive up the entrance ramp to a freeway, you "merge" into the traffic. It's the right verb to use because you do it with care. You'd better!

Subtle difference. Combine means join forces, but not necessarily in such a way that the multiple forces lose their unique identities. They are only acting together. Merge means multiple becomes singular, in such a way that it's no longer clear they were ever separate entities. Combine has the broader meaning, and can include merge, but merge is more specific and does not cover all forms of combination.