deriv LSK ETT STT aSTA ALPH OLDHOMEPAGE NEWHOMEPAGE
The enclitic /tu' means "but".
Sometimes it is used just to change subject, with no "but" meaning, or metri causa. As in —
puruSas sa paraH pArtha bhaktyA labhyas tv ananyayA |
पुरुषः स परः पार्थ भक्त्या लभ्यस्त्वनन्यया ।
where the tu तु just makes the verse fit.
The word tu तु in a sUtra may by this /tu', or may be the /tu meaning "t त् th थ् d द् dh ध् n न्".