Perdere il filo significa distrarsi, disorientarsi mentalmente, e quindi interrompere un discorso, un ragionamento o una conversazione.
To be absent minded
Avere la testa fra le nuvole
To be out of your head/mind
Essere o andar fuori di testa
To get your thoughts in order, think something over
Fare mente locale
To be well versed, knowledgeable on a subject
Essere ferrato
To make up your mind, set your mind on something
Mettersi (qualcosa) in testa
To not have a clue, to not understand a thing
Non capire un cavolo
To lose your train of thought
Perdere il filo
To be a guru, a buff, a know it all
Essere il quinto evangelista
To not have the faintest/slightest idea
Non avere la più pallida idea
To be/keep in the dark, to be unaware
Essere/tenere all’oscuro
To have a memory lapse, to have your mind go blank
Avere un vuoto di memoria
To get the idea from, draw inspiration from, be inspired by, use something as a starting point
Cogliere/Trarre/Prendere (lo) spunto
To be very smart, capable, great
(Essere) in gamba
To have some sense/common sense/to have brains
Avere (del/un po’ di) sale in zucca
Frankly, exactly, without mincing words
Papale, papale
To completely forget/eliminate something
Mettere nel dimenticatoio
To believe/accept with reservations, to take with a grain of salt
Credere/accettare con beneficio di inventario