Jawaban:

Rectifier is an electrical device that convertsan alternating current into a direct one byallowing a current to flow through it in one direction only.

Penjelasan:

convert -> converts

karena kalimat di atas merupakan contoh kalimat Present Tense, yang mana kata kerja yang bertemu subjek It(benda atau kata rujuk)/I(saya)/She(dia perempuan)/He(dia laki-laki) akan ditambah "s" atau "es" diakhiran kata kerjanya.

Misal:

My best friend calls me every night.

bukan My best friend call me every night.

She goes away every August.

bukan She go away every August.

allow -> allowing

karena kata "allow" pada kalimat di atas bermakna "doing something" atau sedang melakukan sesuatu, sehingga kata "allow" di atas ditambahkan "ing" diakhiran kata kerjanya.

Misal:

They are waiting for Cia at my home.

bukan They are wait for Cia at my home.

He is playing soccer with his friends.

bukan He is play soccer with his friends.