Mi costringono ad aspettare.

Mi means me.

In this sentence, it is an unstressed object pronoun, and in Italian these pronouns normally go before a conjugated verb:

• Mi costringono = They force me
• Ti costringono = They force you
• Lo costringono = They force him/it

So mi is the person receiving the action of costringono.

Costringono is the third person plural present indicative of costringere.

So it means:

• they force
• they are forcing in some contexts

The full verb is:

• costringere = to force / to compel

A few related forms:

• costringo = I force
• costringi = you force
• costringe = he/she forces
• costringiamo = we force
• costringete = you all force
• costringono = they force

Italian often omits subject pronouns because the verb ending already shows the person and number.

So:

• Mi costringono ad aspettare
  already tells you the subject is they

You could say Loro mi costringono ad aspettare, but that is usually only for emphasis, contrast, or clarity. In normal Italian, leaving out the subject is more natural.

Not necessarily.

Costringono can refer to:

• a specific group already known from context
• unnamed people
• institutions or circumstances, depending on context

So the subject can be explicitly known, or it can stay vague/unspecified. Italian often leaves that kind of subject unstated if the context makes it clear enough.

Because costringere normally uses this pattern:

costringere qualcuno a fare qualcosa
= to force someone to do something

So the structure is:

• mi = me
• costringono = they force
• ad aspettare = to wait

The a is required by the verb pattern. You are not just placing two verbs together; you need the preposition:

• Mi costringono a partire
• Mi costringono a restare
• Mi costringono ad aspettare

No. Here ad is just a with an added d for sound.

This often happens before a word that begins with a, especially in more careful or traditional writing:

• a aspettare → commonly written as ad aspettare
• a andare → ad andare

So ad aspettare and a aspettare are not different in meaning. The d is only a phonetic/spelling choice.

Not always.

In this sentence, aspettare simply means to wait, and it works fine that way.

But an important difference is that Italian aspettare often takes a direct object where English uses wait for:

• Aspetto l’autobus = I’m waiting for the bus
• Aspetto Maria = I’m waiting for Maria

So English speakers often want to add an extra preposition, but Italian usually does not.

Yes, depending on context.

Italian present tense is often broader than English present simple. It can express:

• a general fact
• a repeated action
• something happening now

So Mi costringono ad aspettare can mean:

• They force me to wait
• They are forcing me to wait

If you want to stress the ongoing action more clearly, Italian can also use:

• Mi stanno costringendo ad aspettare

But the simple present is often enough.

Yes: this is a very useful pattern:

costringere + person + a + infinitive

Examples:

• Mi costringono a partire = They force me to leave
• Ti costringono a scegliere = They force you to choose
• Lo costringono a studiare = They force him to study

So if you learn one thing from this sentence, it is this structure:

someone forces someone else to do something

Yes. A very common alternative is the passive-style expression with costretto:

• Sono costretto ad aspettare = I am forced to wait

This version focuses more on my situation than on who is forcing me.

Compare:

• Mi costringono ad aspettare = They force me to wait
• Sono costretto ad aspettare = I am forced to wait

The second one is especially common when the person or cause is unknown, irrelevant, or left vague.

You just change the object pronoun:

• Ti costringono ad aspettare = They force you to wait
• Lo costringono ad aspettare = They force him to wait
• La costringono ad aspettare = They force her to wait
• Ci costringono ad aspettare = They force us to wait
• Vi costringono ad aspettare = They force you all to wait
• Li costringono ad aspettare = They force them to wait

This is a very common Italian pattern, so practicing it with all the pronouns is useful.