Learn Farsi: من می‌بایست دو قطره از این عصاره را به عطر اضافه می‌کردم. - I should have added two drops of this extract to the perfume.

To express an unfulfilled past obligation (something you should have done but didn't), Farsi pairs می‌بایست (mi-bâyast) with either the Past Continuous or the Perfect Subjunctive. Here, the Past Continuous اضافه می‌کردم (ezâfe mi-kardam) is used to mean 'should have added'.

Could I just use باید (bâyad) instead of می‌بایست (mi-bâyast) to say 'I should have'?

Yes. In everyday spoken Farsi, people often use the present tense باید (bâyad) followed by the past continuous to mean 'should have'. However, می‌بایست is the strictly correct, traditional form for past obligations and is very common in written or careful speech.

Why does the object marker را (râ) come after این عصاره (this extract) rather than right after دو قطره (two drops)?

The entire phrase دو قطره از این عصاره (two drops of this extract) acts as the specific direct object of the verb. In Farsi, را must always go at the very end of the complete noun phrase it is marking.

How is the verb اضافه می‌کردم formed here?

The infinitive is the compound verb اضافه کردن (ezâfe kardan), meaning 'to add'. To form the past continuous for 'I', you take the non-verbal part اضافه, add the continuous prefix می (mi-) to the past stem کرد (kard), and finish with the 'I' ending م (-am).