If you nail down something unknown or uncertain, you find out exactly what it is. It would be useful if you could nail down the source of this tension. = pin down
make final; put the last touches on; put into final form; "let's finalize the proposal"
If you nail down an agreement, you manage to reach a firm agreement with a definite result. The Secretary of State and his Russian counterpart met to try to nail down the elusive accord
birşeyi çok iyi anlamak yada anlatmak, belirgin ve son haline getirmek
الواصلة
bir·şe·yi çok i·yi an·la·mak ya·da an·lat·mak, be·lir·gin ve son ha·li·ne ge·tir·mek