Example (2) is a compound sentence in which the second clause provides additional information about a person or thing which was introduced in the first clase. For example:
Did you see the man who was driving the car?
first clause second clause
We put up the mirror which we bought in the sale.
first clause second clause
Alternative forms of 'bod' are possible for the third person singular present tense.
If the subject of the second clause refers back to a person or thing in the first clause, then the parts of the sentence should be linked with 'sydd'. For example:
Dyma'r teulu sydd yn symud i mewn i'r tŷ.
This is the family who are moving into the house.
The family mentioned in the first clause is carrying out the action of 'moving into the house' (i.e. they are the subject) in the second clause.
Rydyn ni'n siarad â'r dyn sydd yn torri'r lawnt.
We are speaking to the man who is cutting the lawn.
The man mentioned in the first clause is carrying out the action of 'cutting the lawn' (i.e. he is the subject) in the second clause.
If the object of the second clause refers back to a person or thing in the first clause, then the parts of the sentence should be linked with 'y mae'. For example:
Dyma'r tŷ y mae'r teulu'n ei brynu.
This is the house that the family is buying.
The house mentioned in the first clause has the action of 'being bought' carried out on it (i.e. it is the object) in the second clause. The subject of the second clause is the family, who are doing the action of 'buying'.
Canmolodd yr adroddiad y mae'r rheolwr wedi'i ysgrifennu.
He praised the report that the manager has written.
The report mentioned in the first clause has the action of 'being written' carried out on it (i.e. it is the object) in the second clause. The subject of the second clause is the manager, who is doing the action of 'writing'.