La pioggia ci costringe a tornare.

Here, ci means us.

It is an unstressed object pronoun. With the pattern costringere qualcuno a fare qualcosa, the person being forced is the object of costringere. So:

La pioggia ci costringe...
= the rain forces us...

In other contexts, ci can also mean there, but not in this sentence.

In Italian, unstressed object pronouns normally come before a conjugated verb. So you get:

ci costringe

not:

costringe ci

This is the standard position for pronouns like mi, ti, ci, vi, lo, la, li, le when they go with a finite verb.

The subject is la pioggia, which is third person singular, so the verb must also be third person singular:

• costringere = infinitive
• costringe = he/she/it forces

Since la pioggia is singular, costringe is the correct form.

Because costringere normally uses the pattern:

costringere qualcuno a + infinitive

So you say:

• costringere qualcuno a partire
• costringere qualcuno a restare
• costringere qualcuno a tornare

Without a, the sentence would be incorrect.

After costringere a, Italian uses an infinitive, not a second conjugated verb.

So the structure is:

La pioggia + ci costringe + a tornare

The action that is being forced is expressed with the infinitive tornare. English often works similarly with to return after forces us.

It is a direct object here.

The pattern is:

qualcuno costringe qualcuno a fare qualcosa

The person being forced is the direct object of costringere. So ci stands for noi as a direct object, not to us.

Yes, often you could. Tornare and ritornare can both express the idea of returning. In everyday Italian, tornare is extremely common and sounds completely natural.

Depending on the situation, another verb like rientrare might also be possible, but that can suggest returning home or going back inside, so it is not always interchangeable.

No. Italian, like English, often lets an inanimate thing be the subject of a verb like costringere. It means the circumstances created by the rain leave us no real choice.

So it does not have to mean physical force. It can simply mean that the rain makes it necessary.

Yes, but the given order is the most neutral and natural:

La pioggia ci costringe a tornare.

Italian does allow other word orders for emphasis, but they are less neutral. For a learner, this standard order is the best one to use:

subject + object pronoun + verb + a + infinitive