The theme of "A lesson in love" is more or less the same as the theme of the "Pina Colada" song of Rupert Holmes (1979). A man and a woman have written off their marriage after 15 years and are both hunting for a new partner. During the hunt they rediscover each other.
As the theme already indicates the film has two types of scenes.
Scenes with respect to the worn out marriage (tragedy).
Scenes with both spouses (but primarily the man) on partner hunt (comedy).
As Bergman says at the beginning of the film: "This is a comedy that could have been a tragedy".
To be honest, the comedy element of this film is rather bad. It is old fashioned, over the top and maybe both. Only a year later Bergman would prove that he knows how to handle a comedy with "Smiles of a summernight" (1955).
With respect to the theme of the film the low quality if the comic scenes proves to be a blessing in disguise. Despite all the fights the family scenes are more sincere and even more hopeful than the flirtation scenes. After all to argue with someone means that you care for someone.
By the way, the real lesson in love is given by the grandparents. They show how to live with the imperfections of your partner without hurting his / her feelings or self esteem. They do so already 50 years.