Why is it ‘ne devrions fumer plus’ when there is a conjugated and an infinitive?

Hi Sue,

What is your question exactely ? Is it about the verbal forms?

The correct way to say it is 'Nous ne devrions plus fumer' (Nous ne devrions plus + inf)
Do we say 'She shouldn't smokes anymore' ? 
Je te laisse réfléchir à tout ça (I let you think!)