When a part of speech other than an adverb is used as an adverb in Hindi, it is typically placed in the masculine singular oblique (MSO) form. It is important to recognize that the oblique case has many uses. Although indicating the object of a postposition is the most common use of the oblique case, it is by no means the only use.
Consider a few examples:
वह औंधे मुंह गिरा – “He fell flat on his face” (literally, but this is used as a metaphor for shame)
In the previous example, the noun मुंह (face) was in its MSO form; although it is an invariable masculine noun, the adjective was in MSO form (औंधे).
मैं अगले महीने भारत जाने वाला हूं – “I’m going to go to India next month”
In the previous example, the noun महीने is in MSO form, as well as the adjective अगले.
मैं पिछले साल नेपाल गया था – “I went to Nepal last year”
In the previous example, the phrase पिछले साल was in MSO form.