Comma conundrum

In Danish, you often put a comma before ‘at’ (subordinate conjunction) and ‘som/der’ (relative pronoun):

• Han sagde, at han ville komme.
• Cillian Murphy, der spiller hovedrollen i Peaky Blinders, skal måske erstatte Daniel Craig som James Bond.

This is not the case in English.

1. Before subordinate conjunctions
In English, we do not place a comma before ‘that’ when it is used as a subordinate conjunction (words that connect a main clause with a subordinate clause) as in the first example above.

• He said that he would come.
• I decided to go to work on the off-chance that someone had brought bread for breakfast.

In other words, in English, you avoid separating the main clause and the subordinate clause.

2. Before relative pronouns
It becomes somewhat more complicated and less straightforward when turning to the question of whether one should put a comma before ‘that’ when ‘that’ functions as a relative pronoun.

In short, you do not put a comma before ‘that’ in English, since ‘that’ introduces a restrictive relative clause, which never requires a comma. A restrictive relative clause provides vital information about the word that ‘that’ refers back to; if removed, the sentence is incomplete:

• She wore the glasses that she bought on sale.
• She bought tickets for the Foo Fighters concert that takes place in Horsens.

3. Parenthetical relative clauses
If a relative clause is parenthetical, i.e. merely offers additional information about the word it modifies instead of defining/identifying it, a comma is required before the relative pronoun. A useful rule of thumb is to remove the relative clause; if the sentence still makes sense, one should place a comma. If the relative clause is parenthetical, ‘which’ must be used as the relative pronoun instead of ‘that’:

• The apple-printed pants, which she bought online, fit her nicely.
• Elia, which was inaugurated in 2001, last erupted 10 March 2019 at 19:05.