Does merkle root contain hashes of transactions from previous blocks? Every block in a blockchain contains merkle root value. So for any block, there is some list of transactions and you create a merkle tree based on these transaction.
For other blocks, same process will repeated.

My question is:
Let us assume that A, B,C,D are transactions and we create a merkle tree and we add the merkle root in block. Now later we got the transactions E,F,G,H so:


whether merkle tree is based on E,F,G,H only
or it is based on A,B,C,D,E,F,G,H .


I want to know that for some block, we created a merkle root value based on A,B,C,D. For others block whether we include previous transaction with new transactions  and create a merkle tree. Or with only new transactions, merkle tree is created.
The blockchain is the mechanism for Bitcoin to converge on one shared state. Participants submit payment orders in the form of unconfirmed transactions to the network from which miners then select a set of transactions into their block candidates. The Merkle root hereby represents a commitment to one specific list of previously unconfirmed, valid transactions. When a new block gets authored, all network participants validate the block and then adopt the new set of balances as the current network state. 

Beside the Merkle root as a commitment to this block's transactions, the block header also commits to exactly one preceding block as its parent by including the hash of that predecessor block. As the predecessor in turn committed to it's own predecessor, a new block therefore inductively commits to the full history since the Genesis Block.

Given that each preceding block in turn contains a Merkle root with which it committed to its own set of transactions, any new block also commits indirectly to the full history of all transactions.

Coming back to your example, this means that if Block#1 committed to {A, B, C, D}, the Merkle root in Block#2 would only commit to {E, F, G, H}. However, Block#2 commits to {A, B, C, D} indirectly by declaring Block#1 it's predecessor.