The past subjunctive (viðtengingarháttur þátíðar) is used for hypothetical situations ("if X, then Y") and polite requests. In hypotheticals, both the condition ("if" clause) and the result use the past subjunctive.
For many weak verbs, the past subjunctive is identical to the regular past tense (like smíðaði and virkaði).
However, many verbs undergo a vowel shift (i-mutation) in this mood. Singular endings are typically -i (ég, hann/hún/það) and -ir (þú). For example, the a in hafa (to have) becomes e (hefði):
| If the genius had the idea, he would build the invention. | Ef snillingurinn hefði hugmyndina, smíðaði hann uppfinninguna. |
| If the glider had a motor, it would work better. | Ef svifflugan hefði mótor, virkaði hún betur. |
Polite Requests
The past subjunctive is essential for polite questions. The verbs mega (may) and geta (can) shift to mætti and gæti. (Note that gætir þú is frequently contracted to gætirðu).
| May I see the invention in the laboratory? | Mætti ég sjá uppfinninguna á rannsóknarstofunni? |
| Could you fix the motor for me? | Gætirðu lagað mótorinn fyrir mig? |
Common Vowel Shifts
Here are common vowel changes used to form the past subjunctive:
- a becomes e: að segja (to say) → segði
- á becomes æ: að eiga (to have/own) → ætti
- u becomes y: að þurfa (to need) → þyrfti
- ú becomes y: að trúa (to believe) → tryði
Notice how these shifted verbs appear alongside regular weak verbs (like gerði and reyndi) in the following hypotheticals. For plural subjects, the subjunctive verb takes the ending -u (tryðu).
| If she did a new experiment, she would need the patent. | Ef hún gerði nýja tilraun, þyrfti hún einkaleyfið. |
| If the brave man had the money, he would try to fly. | Ef hugrakki maðurinn ætti peninginn, reyndi hann að fljúga. |
| If the teacher told the story, the students would believe him. | Ef kennarinn segði söguna, tryðu nemendurnir honum. |