What do you understand by a ‘Competent Witness’? Are husband and wife competent witnesses to give evidence against each other in a criminal case?

Witness has an important place in the law of evidence. It is the witness on whose statements court relies to decide a case. Hence, the witness must be competent. 

Under Section 118 Who may testify has been explained. —All persons shall be competent to testify unless the Court considers that they are prevented from understanding the questions put to them, or from giving rational answers to those questions, by tender years, extreme old age, disease, whether of body or mind or any other cause of the same kind. 
Explanation — A lunatic is not incompetent to testify unless he is prevented by his lunacy from understanding the questions put to him and giving rational answers to them.
Minor person If a witness is 14 years of age or older, they have to give evidence on oath or affirmation. Children under 14 years of age do not have to swear an oath or make an affirmation before giving evidence.
In some cases involving sexual offenses, the judge can decide not to let members of the public into the court.
There is no definite age determined for the competency of a witness. 

In the case of Prakash vs the State of MP 
Under the case of murder, the witness was given by a 14-year-old younger brother of the deceased. It was said that the minor cannot be a competent witness, as his evidence may be taught. But, the Supreme Court held him to be a competent witness. 

Dumb Witness 
Under section 119 Dumb witnesses were explained
A witness who is unable to speak may give his evidence in any other manner in which he can make it intelligible, as by writing or by signs; but such writing must be written and the signs made in open Court. The evidence so given shall be deemed to be oral evidence.

Husband and wife 

Under section 120 Parties to a civil suit, and their wives or husbands have been given. 
Husband or wife of person under criminal trial.—In all civil proceedings the parties to the suit, and the husband or wife of any party to the suit, shall be competent witnesses. In criminal proceedings against any person, the husband or wife of such person, respectively, shall be a competent witness.
Example – In a suit for dissolution of marriage on the ground of adultery, the evidence of the husband or wife that they did not have access to each other when she was begotten, shall be admitted to prove adultery.