Create a sentence with an inverted word order.

in the garden were all his friends to welcome him home.

In the garden were all his friends to welcome him home.

Using an adverbial of place at the start (for emphasis) allows you to use the inverted structure.

All his friends were in the garden to welcome him home.

This is a correct, normal sentence where the subject (All his friends) comes before the verb (were). So for this excercise this is not what we were looking for.

To welcome him home, all his friends were in the garden.

This is a compound sentence with the dependent clause at the start (To welcome him home). It needs a comma in the middle. The main clause is a full, normal sentence where the subject (all his friends) comes before the verb (were). So for this excercise this is not what we were looking for.

To welcome him home, in the garden were all his friends.

This is a compound sentence with the dependent clause at the start (to welcome him home). As the emphasis is on to welcome him home, the adverbial of place (in the garden) is no longer used for emphasis and should take its natural position at the end of the sentence.

To welcome him home, all his friends were in the garden.

Correct English, but not what we were looking for (not an inverted construction). This is a compound sentence with the dependent clause at the start (to welcome him home). As the emphasis is on to welcome him home, the adverbial of place (in the garden) is no longer used for emphasis and should take its natural position at the end of the sentence.