Rules of substitution PF to IPF after modal verbs with negation in Russian

This paper deals with the cases in Russian where the perfective aspect (PF) is, or is not,
replaced by the imperfective aspect (IPF) after a modal verb with negation: dolzhen kupit’ (PF)
‘must buy’ — ne dolzhen pokupat’ (IPF) ‘must not buy’, but mozhet kupit’ (PF) ‘can buy’ — ne
mozhet kupit’ (PF) ‘cannot buy'. It shows that in most contexts this substitution is governed by
one rule: if the subject of modality controls the action, PF is changed to IPF; if not, PF is retained.
Hence, the change normally occurs in deontic contexts, when the subject has a full
control over the situation, and does not occur if the control is only partial. Epistemic contexts
and expressions of internal modality do not allow the substitution. Expressions of will allow
both possibilities.
The same rule is acting in other parts of Russian grammar, including the well-known case
of negated imperatives.
The main exception from the rule of controllability is that PF can be retained if the absence
of action contradicts the speaker’s expectations. This is also paralleled in the so-called
“contrary-to-expectations present”.
The paper also discusses a number of particular cases where the expected substitution is
blocked.