http://forum.mikrotik.com/t/how-do-i-connect-two-subnet-in-a-single-router/149158/21
This link might help you. Basically, all ethernet ports are attached to a bridge. You detach your desired ethernet port from that bridge. Make a DHCP server for that port,add ip addresses and define ip pool. Also there is a video on YouTube by Tksja , https://youtu.be/6G6dWF1nipY
Here is the break down of subnet creation:
Remove the desired port from the default bridge(for my case its ether5):
/interface bridge port remove [find interface="ether5"]
This portion is firewall dependent, my firewall requires that it should be in “LAN”
/interface list member add interface=ether5 list=LAN
Now, Give Ip address to that ethernet port.
/ip address
add address=192.168.188.1/24 comment=pinet interface=ether5 network=192.168.188.0
Assign an IP range for the subnet.
/ip pool
add name=pinet ranges=192.168.188.20-192.168.188.40
Add a dhcp server for that newly created subnet and dhcp server network.
/ip dhcp-server
add address-pool=pinet disabled=no interface=ether5 name=pinetDHCP
/ip dhcp-server network
add address=192.168.188.0/24 comment=pinet gateway=192.168.188.1 netmask=24
If you need more clarification or help, please post on the above mentioned thread, there are other more advance/experienced members who might help 