Graduate studies Kuiper continued his studies in classical literature at Leiden. There, he studied under
Uhlenbeck, but following Uhlenbeck's retirement, he began studying under
Nicolaas van Wijk, a profound influence on Kuiper's work; following Kuiper's death,
Henk Bodewitz described Van Wijk as Kuiper's "great inspirator". Kuiper studied the
Balto-Slavic languages with Van Wijk, including Russian, Lithuanian, and
Old Church Slavonic. In Van Wijk's posthumous , Kuiper described the change from Uhlenbeck to Van Wijk thus: In 1934, Kuiper completed his , which granted him the equivalent of a
master's degree, in classical literature; just two weeks later, he successfully defended his
dissertation, receiving his
doctorate . The time between these two events was short. His degree was funded by his military service on the condition that he teach classics at a
lyceum in the
Dutch colony of
Batavia in the
East Indies (modern-day
Jakarta, Indonesia) immediately upon graduation. Kuiper consequently postponed his in order to get his doctorate before departing for the Dutch East Indies, thereby securing his thesis defense and earning both his and his doctorate before he was forced to honor the conditions of his scholarship. He later published an expanded version of his thesis in 1937, which focused on the
nasal presents – that is, the
infixation of the
nasal consonant to form some
present tense verbs – in Sanskrit and other
Indo-European languages.
Life abroad and return to Leiden , as a condition of his military scholarship.|alt=A triangular shaped building with many windows with its view slightly obscured by a massive tree Shortly before departing to Batavia, Kuiper married Eduarda Johanna "Warda" de Jong in 1934. Once there, he taught classics at the Carpentier Alting Foundation Lyceum (). During his time abroad, he continued his research and published several articles. He remained in military service as a reservist, being promoted to
second lieutenant in February 1935 followed by a promotion to
first lieutenant in December as a part of the
Motorized Artillery Regiment. In 1937, he became a member of the
Royal Netherlands Academy of Arts and Sciences. In April 1939, Kuiper resigned his membership. In the fall of the same year, Kuiper was appointed by Leiden University to take over the Sanskrit
professorship after
Vogel's departure, taking the official title of "Regular Professor of Sanskrit and Its Literature and Indian Archeology" (). He gave an inaugural speech shortly after his arrival, entitled "" ('The Divine Mother in the Pre-Indian Religion'). In 1940, he was
honorably discharged from military service as a first lieutenant. Due to the threat of
Nazi Germany at the time, Kuiper returned to military service and taught only once a week, but following
the defeat of the Netherlands and
its subsequent occupation, the university was shut down. Around this time, Van Wijk and he were close friends. When Van Wijk died unexpectedly in 1941, Kuiper was selected as one of the only two to speak at the packed funeral and had a significant role in handling his
estate. Throughout his career, Kuiper kept a portrait of Van Wijk on his desk. Following Van Wijk's death, Kuiper was appointed to take over as
chair of the
Balto-Slavic languages. In 1942, Kuiper published one of his most influential pieces: "Notes on Vedic Noun-Inflexion". The piece argues for a system of two
accent-based
inflection systems in Proto-Indo-European, based on his previous publication ('The Latin Fifth Declension') and the earlier work of the Danish linguist
Holger Pedersen. The article was well-received and was quickly accepted by fellow linguists.
Post-war career Kuiper resumed teaching during the 1945–1946 academic year. During the occupation, he had spent most of his study on the
Munda languages, culminating in an English-language book –
Proto-Munda Words in Sanskrit – published in 1948 which he later described as "immature". The same year, he was again honorably discharged from military service and was re-inducted into the Royal Netherlands Academy of Arts and Sciences. During this period, Kuiper began teaching comparative
Indo-European linguistics, as well as
Old Iranian and
Tamil, and took an academic interest in
laryngeal theory. This theory – which proposed that the
Proto-Indo-European language had three consonants of indeterminate guttural
places of articulation which typically developed into vowels in all but one
daughter language – was still not fully accepted by linguists, but Kuiper presented several innovative perspectives, particularly with respect to their
reflexes in
Vedic Sanskrit. In 1955, Kuiper published an article for the Royal Netherlands Academy of Arts and Sciences entitled "Shortening of Final Vowels in the ". In it, he articulated that short vowels in the last syllable of a word followed by a historical laryngeal consonant – that is, one of the three consonant sounds of unknown articulation – led to a
long vowel in Sanskrit if what followed the vowel was a consonant. However, when this vowel–laryngeal combination occurred at the end of a word or utterance, this short vowel remained unchanged. Kuiper attributed this
alternation to the
loss of the laryngeal , or immediately before a pause in speech. This process is now regarded as having occurred in Proto-Indo-European and has attracted study in other
Indo-European languages since Kuiper's death; languages with attested evidence of this process include
Tocharian,
Latin,
Old Norse, and
Ancient Greek. Kuiper's association with this process has led to it being termed "
Kuiper's law". The following year, Kuiper began publishing increasingly about the possibilities of
non-Indo-European substrate languages as etymological explanations of words in some Indo-European languages. Kuiper argued that the Greek word (, 'man'), for example, had a
Pre-Greek origin rather than an Indo-European one. He abandoned the effort until much later based on what he felt was a field-wide reluctance to take non-Indo-European material seriously. In 1957, he began the
Indo-Iranian Journal, an
academic journal focusing on the cultures and languages of the
Indo-Iranians and their descendants, with his colleague and former student
J. W. de Jong. Kuiper took on one last
research assistant in 1969 before retiring in 1972 after his student achieved his doctorate in March. At only sixty-five years old, his retirement was considered early. Growing frustration with academic reforms, which were viewed as unproductive and frustrating, contributed to Kuiper's earlier-than-typical retirement.
Final years Later in his life, Kuiper married Hanna (), whom he was with for over thirty-five years at the time of his death. As he began to age, Kuiper's eyesight deteriorated tremendously to the point where he could no longer drive. He was able to have it corrected through
eye surgery and immediately bought a new
BMW when he could drive again. Although he continued to write in his nineties, his eyesight began to fail again and he came to increasingly have to care for his wife. Despite this, he remained active in academia well into his final years; he described one paper as his "
swan song", but ended up publishing three more thereafter and was at academic events until May 2002. On the morning of 14 November 2003, Kuiper died in
Zeist at the age of 96. His wife died less than six weeks later. Kuiper had five children, though one predeceased him. He is buried in in Leiden. ==Recognition and legacy==