Dropping har/hade in the perfect tense.
The perfect tense is normally created with har + the supine tense, for example han har sjungit/spelat/sprungit (’he has sung/played/run’). The same goes for the pluperfect which is created using the past tense hade + the supine, as in han hade sjungit/spelat/sprungit (’he had sung/played/run’).
Some of you may have noticed sentences such as this one where there is no har.
- Jag kommer att ha ringt henne innan du kommit dit. (’I will have called her before you have arrived there.’)
In subordinate clauses (clauses that begin with conjunctions like innan, eftersom etc. etc.) and especially in relative clauses (clauses that begin with som etc.), the auxiliary verb har/hade can be left out. This is because the form sjungit already tells us that it’s the perfect or pluperfect tense, so the har/hade isn’t really needed.
- Det var det värsta jag (har) hört! (’That’s the worst thing I have heard.’)
- Jag visste inte då att de redan (hade) gett sig av. (’I did not know then that they had already left.’)
- Det är någon (som) jag (har) träffat en gång. (’It’s someone (that) I have met once.’)
- Den man som (hade) varit här på morgonen dök sedan upp senare på dagen. (’The man who (had) been here in the morning showed up later during the day.’)
- Han är mannen som (har) blivit kändis. (He’s the man who (has) become a celebrity.)
This is a bit of a peculiarity of Swedish that was borrowed from German in the middle of the 17th century, then it disappeared from German in the 19th century, but continued to be used in Swedish. It’s mostly used in written language, and less in spoken language, but you will still find it both in colloquial conversations and in law texts.
A few decades ago, a survey showed that ha was left out 87% of the times in the written material, but only 8% in the spoken material. It hasn’t been researched more lately as far as I know, but there hasn’t probably been a huge change.
In the beginning of the 20th century, writers started adding ha to subordinate clauses where it was left out to be clear, but it started to be accepted in the middle of the 20th century, since it was shown that it existed in both written language and colloquial spoken language, but it continued to be added into law texts. Nowadays, it’s accepted to leave it out everywhere but is still mostly common in writing.
There is a similar phenomenon where the infinitive auxiliary ha (i.e. not har/hade) is left out, and this can be done also in main clauses. Ha can be left out if it’s preceded by an auxiliary verb that is not in the present tense or followed by an att.
- Han kunde inte (ha) varit gladare! (’He couldn’t have been happier!’)
- Jag kunde inte (ha) gjort det bättre själv. (’I couldn’t have done it better myself.)
However, if the first auxiliary ’kunde’ is in the present tense, the ha cannot be removed:
- Han kan inte ha gjort det. (’He cannot have done it.)
To conclude, you can always include har/hade in your writing, but it is good to know that it’s often left out by native speakers in subordinate clauses, where they let the bare supine form convey all the information. Both are acceptable.