This is the usage of subjunctive mood.

If it rains tomorrow, we will have to postpone our visit to Yangpu Bridge.

If it rains tomorrow after the inversion, we will have to postpone our visit to Yangpu Bridge.

If can be omitted when the predicate verbs of the virtual conditional sentence contain were, should and had, then the conditional sentence will be inverted, that is, the three words were, should and had are put at the beginning of the sentence and used more in written language.

If he agrees to go there, we will send him.

If he promises to go, we will send him.

If she were here, she would agree with us.

If she were here, she would agree with us.

If he had studied computer, we would have hired him to work here.

If he knew something about computers, we would have hired him to work here.