Jonas was the youngest of five children. He had a sister, Ona, and three brothers, Petras, Antanas and Juozas. In 1944, at the request of priest Vytas Baltutis, the Kurpaitis family took in four people of Jewish origin, three of whom were children at the time - Miriam, Iser, Masha and Esther Gail for six months. The Kurpaitis family hid and cared for these people for six months until the area was occupied by the Soviets in October 1944.

As the front approached, Jonas went into hiding because he didn't want to join the Soviet army. At first, he hid in Kaunas with friends of his relatives. He returned from Kaunas on foot and settled in the village of Mikalauka with his sister Onutė Stankevičienė. The NKVD units rampaged through the villages looking for men. Urged by his teacher E. Malijonis and recommended by the partisans, Jonas and a friend went on foot to the Kelmiškės forest in the early spring of 1945 and joined the partisan detachment led by Jonas Neifeltas. After taking the oath, he was given a hand machine gun. He was given the nickname "Viksva" (sedges).

Recalling those days, Jonas said: "When I went to the partisan deployment site, I found out that my mother had died on 23 March and I could not attend the funeral. Once I dreamt of my mother, who told me that she was waiting for me and couldn't wait for me. When I woke up, I found out that the Russian army had surrounded the Kalniškės forest and we would have to fight. Soon we were at the top of the hill and, having taken up our positions, we were waiting for the enemy to attack. The Russians, not expecting strong resistance, moved almost without hiding. As we got closer to them, we opened rifle and machine-gun fire. The first attack was repulsed, but the Russians were not going to retreat. More and more reserves were sent to the battlefield. From the enemy side there was a constant, fierce roar of automatic weapons and machine guns. Apparently, mortars were used. The hellish shots broke tree branches and young trees. Our position was much better because we were firing from a hill, so the grenades we fired fell far away. This battle continued, with minor interruptions, until late in the evening. Only later did we learn that we were attacked by regular army units returning from the front. We then killed about four hundred of the enemy.

When the ammunition ran out and the evening came, the order was given to retreat. I don't know how many of us were left alive, but seven of us successfully retreated. The next day, stribai (local armed members) and the NKVD units were on the rampage in all the surrounding areas, looking for the remaining partisans.

Not all the farmers accepted us because they were afraid. We split up. After dropping my machine gun (there were no more cartridges), I found a hole in the ground somewhere near the Kirsna stream, in a wet meadow, near some bushes. I lay down in it and covered myself with tree branches and grasses, and mentally crossed myself. I heard shots echoing nearby. The soldiers must have seen my friends and shot them, and then looked for me. Soon the soldiers passed by me. When they saw a fish-bait dropped near me, they said: "Vot sietka dlia lovli rib" ("Here is a net for catching fish"). And they went away without looking. After lying like that for a whole day and making sure that everything had calmed down, I left my hiding place. I went to a resident called Paskevičius, who took me in and hid me. I survived there until September. Then I went to Poland and stayed with relatives of my brother Antanas' wife. There, after surviving without papers until 1947, I was arrested and taken to Gusevas. From Gusevas I was brought to Marijampolė. During the cruel interrogations, I managed not to say that I was a partisan, a participant in the battle of Kalniškės. I convinced the Chekists (employees of the USSR security institutions) that I was avoiding serving in the Soviet army. As a result, on 6 January 1949, the court sentenced me to twenty-five years' imprisonment as a "traitor to the fatherland". At the Vilnius transfer station I received news from my relatives that my father had died. I did not have to attend my mother's funeral, and the same happened when my father died."

Jonas Kupraitis was sent to the Komiya camps through the transfer stations. Here, about two hundred male prisoners were housed in old barracks in the forests. All of them worked in logging. They were supervised by armed guards with dogs. In the spring, when the rivers flooded, they worked in the woods.

Prisoners began to write to Moscow and to the Lithuanian prosecutor's office about the wrongful conviction. Only in 1955, after Stalin's death, was a reply received that Jonas Kupraitis was free to return to Lithuania. After six years of prison labour, Jonas returned to his homeland in July 1955. He settled in the village of Milakauska with his sister Onutė Stankevičienė. He registered, got his passport and worked in the field brigade of the "Banga" collective farm.

In 1960 Jonas married Anelė Puzaraitė. The family had two daughters who gave Jonas and Anele 3 granddaughters.