[english]

A poor nucleophile cannot sufficiently displace the leaving group.  The leaving group dissociates...via an SN1 mechanism.  A carbocation intermediate will be formed...of which possible rearrangements can occur, as you mentioned...which entails you are referring to an SN1 mechanism.

[deutdeut]

Not all nucleophiles are bases, why? Isn't they are all Lewis bases as they act as electron donor to form covalent bond?

[english]

Not all nucleophiles are bases, why? Isn't they are all Lewis bases as they act as electron donor to form covalent bond?

Lewis bases, yes.  We mean Bronsted-Lowry bases.  An alcohol can be a nucleophile...it is a Bronsted-Lowry acid, but a Lewis base.

[enahs]

A poor nucleophile cannot sufficiently displace the leaving group.  The leaving group dissociates...via an SN1 mechanism.  A carbocation intermediate will be formed...of which possible rearrangements can occur, as you mentioned...which entails you are referring to an SN1 mechanism.

In a secondary substrate elimination (E2) is favored by a strong base due to steric hindrance.

[deutdeut]

So we are mentioning the bases are strong or weak, it just depends on how well they share electrons with a proton? (by Lewis base definition)  And Mr. k.v. , when using weak nucleophile with a secondary halide, say , if water or alcohols reacts with secondary halides, hydrolysis occurs,  which mechanisms, SN1 or E dominates?