Dia daoibh 'chuile dhuine. Táim ag léamh píosa ar Tuairisc.ie agus tháinig mé ar an abairt seo...

"Is i mBéarla a bheas na léachtaí ar siúl..."

What I am wondering p is, do I HAVE to use "a bheas" here because it's a relative clause? Or can I ise "a bheidh" instead?

GRMA

You're certainly not required, because the Caighdeán doesn't use it. Neither does, I believe, Munster Irish. However, both Connacht and Ulster still use it. Connacht, however, does have a different present tense form than Ulster.

I think you can find "a bheas" in the Caighdeán grammars (New Irish Grammar, Graiméar Gaeilge na mBráithre Críostaí), even though it's not a mandatory form.

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

Go raibh maith agaibh lads

Also, since I just realized I mentioned the unique Connacht form, but didn't show it: basically, you just add an <-s> to the end of the non-relative form:

An fear a bhíonns anseo; na mná a théanns go dtí an leabharlann, etc.