The Difference Between "たら" and "と" (Revised)

このボタンを押したら水が出ます。 If you press this button, water will come out.
このボタンを押すと水が出ます。 If you press this button, water will come out.

The meanings of the above sentences are very close. There seems to be only one difference, which is grammatical: the former connects to the past form of verbs, whereas the latter connects to the present form. In fact, however, there is a major grammatical difference between the two, at least in origin: the former is a conditional sentence, whereas the latter is not. You can see this difference when you make some sentences expressing your volition or desire. For example, you cannot replace "たら" with "と" in the following sentences because the main clause "結婚しよう" or "結婚したい" expresses the volition or the desire of the speaker, which require a conditional clause.

大人になったら結婚しよう。 (Correct) Let’s get married when we grow up.
大人になると結婚しよう。 (Incorrect) Let’s get married when we grow up.

大人になったらさっちゃんと結婚したい。 (Correct) I want to marry Satchan when I grow up.
大人になるとさっちゃんと結婚したい。 (Incorrect) I want to marry Satchan when I grow up.