Function of "だと"


In this sentence:「 これだと乗り換えなし。」I understand that the speaker is trying to tell
me that there is no need to change trains (interchange) when riding a certain
train.

What is the function of 「だと」in this sentence? Somethimes i hear 「だって」in a
rather forceful/insistent tone. Are they the same? Or their meanings are
totally differenf?




> これだと乗り換えなし。  
>  What is the function of 「だと」in this sentence?

The だと consists of the copula 「だ」 (or, the assertive auxiliary / 断定の助動詞「だ」)
and  
the conjunctive particle / 接続助詞「と」, meaning "if" or "when".

So これだと literally means "If/When (it) is this", ⇒ "If/When you choose/take/use
this", or "With this".

You could rephrase your sentence as これ **なら** 乗り換えなし. "If it's this, ⇒ With
this, you don't have to change trains."

> Sometimes I hear 「だって」 in a rather forceful/insistent tone. Are they the
> same?

No, the だと here is quite different from だって.

The だと that can be rephrased as だって, as in 「なん **だって** ？」≂「なん **だと** ？」, is
the copula 「だ」 + the quotative particle / 引用の格助詞「と」.

これだって can mean "even if (it) is this" / "even this" (≂ これ **でも** ), or "this
one, too" (≂ これ **も** ). The だって here is made of the copula 「だ」 + the binding
particle / 係助詞「とて」.

