Use the l-participle of сѣсти and the present tense of the auxiliary verb бꙑти e.g. third-person masculine singular → сѣлъ ѥстъ
pluperfect2
Use the l-participle of сѣсти and the imperfective aorist or imperfect3 tense of the auxiliary verb бꙑти e.g. third-person masculine singular → сѣлъ бѣ/бѣаше
Use the l-participle of сѣсти and the future tense of the auxiliary verb бꙑти e.g. third-person masculine singular → сѣлъ бѫдетъ
conditional2
Use the l-participle of сѣсти and the conditional mood5 or perfective aorist tense of the auxiliary verb бꙑти e.g. third-person masculine singular → сѣлъ би/бꙑ
1 For declension of participles, see their entries. 2 Except first/second-person neuter. 3 Third-person only. 4 Attested only in a few instances. 5 The dual forms are unattested. 6 Also called a new aorist. 7 Also called an asigmatic or simple aorist.
А. К. Поливанова, editor (2013), “Глагол сѣсти”, in Старославянский язык. Грамматика. Словари.[1] (in Russian), Moscow: Университет Дмитрия Пожарского, page 343
“сѣсти”, in Исторически речник на българския език [Historical Dictionary of the Bulgarian Language] (in Bulgarian), Sofia University "St. Clement Ohridsky", 2011–2022