I was, she was
Ten minutes of building past-tense sentences word by word, so the auxiliary lands in second place by habit rather than by rule - and so the participle ending stops being a decision and starts being a reflex.
The part of the grammar page this lesson drills — read it here, then use it.
Overview
Serbian builds the past tense from two words: the short present of бити - сам, си, је, смо, сте, су - plus a participle, a special form of the main verb. Ја сам радио is literally “I am worked” - an auxiliary that never changes its meaning, plus a participle that carries the verb.
This one form does the work of several more specific past tenses - context carries the finer distinction, so at A1 you can use it for any finished action in the past.
Making the participle
Take the infinitive, drop -ти, and add the ending - -о for a male subject, -ла for female, -ло for neuter; see the table below. Nearly every verb you have met so far works this way.
In the plural: -ли for men or a mixed group, -ле for a group of women, -ла for neuter things. A mixed group always takes the masculine -ли, even if only one man is present.
The participle agrees with the subject
The ending is chosen by who or what the subject is, not by the person speaking to you. Ана је читала and Марко је читао use the same auxiliary је and differ only in the participle - the part that carries the gender.
This includes ја and ти, which have no gender of their own: a woman says ја сам радила and a man ја сам радио - the sentence itself reveals who is speaking.
| Person | Singular | Plural |
|---|---|---|
| 1. | ja sam | mi smo |
| 2. | ti si | vi ste |
| 3. | on je | oni su |