A sad face

means, incorrect. A happy face

means, correct.
1. At that time he
is elected to be a Conservative.
==>'that' refers to the past--a location in time far from the speaker. "is" refers to the present.
2. At that time he
was elected be a Conservative."
==> Both 'that' and 'was' refer to the past.
3. I have
experienced that before.
==> 'have experienced', a present prefect verb form, does not express when in the past the event (experience) happened. Use the present perfect when you want to place 100% focus on the event; Use the simple past (i.e. I experienced that before), when you want to place 50% focus on when it happened and 50% focus on the event.
4. I have
experience that before.
==> The verb 'experience' needs '-ed' because it is part of a present perfect verb: 'have -ed/-en'.
5. I have undergone that before.
==> The verb 'undergone' doesn't take the past marker -ed because it already has a past form: under
go (present), under
went (past), under
gone (past participle). 'have under
gone' is the perfect form of the verb "under
go".
6. Pual remarries a clever wife
named Denyse.
==> The word 'named' is an adjective. "remarries" is the verb. There can only be one verb per sentence in English.
Paul (Subject)
remarries (Verb)
a clever wife (Object)
named Denyse (Object Complement; function: describes the noun 'wife')
All the best,