Python 3, 154 bytes

M=input();Q={'ser':'er','ver':'veI','ir':'ib'}.get(M);print([(Q if Q else M[:-2]+("ab"if M[-2:]=="ar"else"I"))+j for j in"a as a amos ais an".split(' ')])

Ungolfed

M=input()
Q={'ser':'er','ver':'veI','ir':'ib'}.get(M)

H=[]
for j in "a as a amos ais an".split(' '):
    if Q:
        F = Q
    else:
        if M[-2:] == "ar":
            F = M[:-2] + "ab"
        else:
            F = M[:-2] + "I"
    H += [F + j]

print(H)

Python 3, 154 232 bytes

M=input()
Q={'ser':'er','ver':'veI','ir':'ib'}.get(M)
H=[(Q if Q else M[:-2]+("ab"if M[-2:]=="ar"else"I"))+j for j in"a as a amos ais an".split(' ')]
if M[-2:]=="ar":H[3]=M[:-2]+"Abamos"
if Q in['er','ib']:H[3]=H[3].title()
print(H)

Fixed the missing capitals in irregular verbs.

Ungolfed

M=input()
Q={'ser':'er','ver':'veI','ir':'ib'}.get(M)

H=[]
for j in "a as a amos ais an".split(' '):
    if Q:
        F = Q
    else:
        if M[-2:] == "ar":
            F = M[:-2] + "ab"
        else:
            F = M[:-2] + "I"
    H += [F + j]

if M[-2:] == "ar":
    H[3] = M[:-2] + "Abamos"

if Q in ['er', 'ib']:
    H[3] = H[3].title()

print(H)

Python 3, 154 bytes

M=input();Q={'ser':'er','ver':'veI','ir':'ib'}.get(M);print([(Q if Q else M[:-2]+("ab"if M[-2:]=="ar"else"I"))+j for j in"a as a amos ais an".split(' ')])

Ungolfed

M=input()
Q={'ser':'er','ver':'veI','ir':'ib'}.get(M)

H=[]
for j in "a as a amos ais an".split(' '):
    if Q:
        F += Q
    else:
        if M[-2:] == "ar":
            F = M[:-2] + "ab"
        else:
            F = M[:-2] + "I"
    H += F + j

print(H)

Python 3, 154 bytes

M=input();Q={'ser':'er','ver':'veI','ir':'ib'}.get(M);print([(Q if Q else M[:-2]+("ab"if M[-2:]=="ar"else"I"))+j for j in"a as a amos ais an".split(' ')])

Ungolfed

M=input()
Q={'ser':'er','ver':'veI','ir':'ib'}.get(M)

H=[]
for j in "a as a amos ais an".split(' '):
    if Q:
        F = Q
    else:
        if M[-2:] == "ar":
            F = M[:-2] + "ab"
        else:
            F = M[:-2] + "I"
    H += [F + j]

print(H)