Sentences using the phrase “supposed to” are generally said in Marathi as if saying “want to”.
e.g. I am supposed to meet him tomorrow -> I am want to meet him tomorrow -> मला त्याला उद्या भेटायचे आहे ( malA tyAlA udyA bheTAyache Ahe)

Such sentences are discussed in this lesson.
Using “To Want”/”To Need” with other verbs

However, sometimes you can find that sentences with “supposed to” are said little differently. Sentence structure is similar to the sentence with “to want”; the only difference is in the form of the subject. As highlighted below.

Form of subject with preposition "To"	object	Main verb + form of आयचा(AyachA) as per gender and multiplicity of noun	form of “असणे”(asaNe) as per object and tense
त्याला	आंबा	खायचा	होता

Form of noun/pronoun as in simple past tense	object	Main verb + form of आयचा(AyachA) as per gender and multiplicity of noun	form of “असणे”(asaNe) as per object and tense
त्याने	आंबा	खायचा	होता

The form of आयचा (AyachA) is created based on Gender and multiplicity of the object; similar to what we have seen in Adjectives.
i.e. it follows आ-ई-ए-ए-या-ई (A-I-e-e-yA-I) rule

All forms are mentioned in
Using “To Want”/”To Need” with other verbs

For अकर्मक verbs i.e. verbs which do not need a noun, the for of आयचा (AyachA) will always be neuter singular i.e. आयचे (Ayache).

e.g.
I am supposed to finish that bottle
Here noun is a bottle that is feminine singular. So the form of आयचा (AyachA) is आयची (AyachI).
To Finish = संपवणे (saMpavaNe)
So verb form will be संपव(saMpav) + आयची (AyachI)= संपवायची(saMpavAyachI)

Full-sentence will be
I am supposed to finish that bottle -> मी ती बाटली संपवायची आहे (mI tI bATalI saMpavAyachI Ahe)

Changing the subject will not change the rest of the sentence as it depends on the object.

He is supposed to finish that bottle: त्याने ती बाटली संपवायची आहे (tyAne tI bATalI saMpavAyachI Ahe)

She is supposed to finish that bottle: तीने ती बाटली संपवायची आहे (tIne tI bATalI saMpavAyachI Ahe)

In the past tense, the only form of the verb will change.

I/He/She was supposed to finish that bottle: मी/ त्याने/ तीने ती बाटली संपवायची होती (mI/ tyAne /tIne tI bATalI saMpavAyachI hotI)

Changing object multiplicity sentence will be
I/He/She was supposed to finish those bottles : मी/ त्याने/ तीने त्या बाटल्या संपवायच्या होत्या (mI/ tyAne /tIne tyA bATalyA saMpavAyachyA hotyA)

For अकर्मक verbs, the form is always आयचे (Ayache).
e.g.
I am supposed to go there tomorrow: मी उद्या तिकडे जायचे आहे (mI udyA tikaDe jAyache Ahe)

In the present tense, the form of असणे(“asaNe”) can be omitted in spoken language
e.g.
He is supposed to come tomorrow:
त्याने उद्या यायचे आहे (tyAne udyA yAyache Ahe )
त्याने उद्या यायचं (tyAne udyA yAyachM )

Such sentence formation can be used when you are urging someone affectionately.
e.g.
This time we could not meet but must meet next time Or This time we could not meet, but next time we are supposed to meet for sure. -> यावेळी भेटू शकलो नाही पण पुढच्या वेळी नक्की भेटायचं (yAveLI bheTU shakalo nAhI paN puDhachyA veLI nakkI bheTAyachM)

The popular use of this form is found on the wedding invitation card. Where you find youngsters of family urging the invitee to attend the function.

e.g.
Do attend my uncle’s wedding – chintu, motu, gotu
माझ्या काकाच्या लग्नाला यायचं हं – चिंटू, मोटू, गोटू
(mAjhyA kAkAchyA lagnAlA yAyachM haM – chiMTU, moTU, goTU)

Such sentence formation can be used when you are forcing or threatening someone.
e.g. You must pay me ransom tomorrow itself
तू मला उद्याच्या उद्या खंडणी द्यायची (tU malA udyAchyA udyA khMDaNI dyAyachI )

Try creating sentences with different verbs using above rules. Crosscheck them by creating same by “verb forms” feature on https://learnmarathiwithkaushik.com/