I've searched for some information about frame structure in wifi and ethernet in the Data Link layer and found out that the two frames look quite different. I am wondering if there is some method in the router that converts wifi frame to ethernet frame in a such network where wireless network is connected to wired network, like WLAN. Since I'm really new to this field, I may have some misunderstanding issues. Thank you.
Connecting different or similar networks on the data link layer (L2) is done by a network bridge - you extract relevant data from an ingress frame (esp. source and destination MAC addresses), create a new frame for egress and copy the relevant data into it.
A bridge also examines each frame's source MAC address to learn where each node is located, so it only copies a frame to the other side when its destination is located there.
However, a router works on the network layer (L3): it forwards by IP destination address between IP networks/subnets. It uses data link layer (L2) encapsulation for reaching the next hop but doesn't convert anything. (It removes the encapsulating frame from a received packet, decides where to forward the packet, and then creates a new frame for that packet.)
You might be referring to a consumer-grade Wi-Fi router which effectively contains a NAT router, an Ethernet switch, a wireless access point (mostly bridged), DHCP and DNS servers, etc, blurring the distinction between all those functions.