Fòram na Gàidhlig

From my understanding, the pronoun e stands for he/him/it and the pronoun i stands for she/her/it. In the context of It is, will it ever be tha i or will it always be tha e?

One of those "depends" answers coming up

The short answer is no. In conservative grammar, if you refer to anything in the 3rd person, you go by grammatical gender. That means that:
Tha an uinneag mòr > Tha i mòr
Tha am bòrd mòr > Tha e mòr
etc

However. In everyday Gaelic, this distinction has been largely levelled with independent pronouns. This means that unless the object is animate (people or animals) and perhaps very core concepts like grian, gealach, sìde and countries (all i) native speakers usually default to e:
Tha an uinneag mòr > Tha e mòr
Tha am bòrd mòr > Tha e mòr
Tha Màiri an-seo > Tha i an-seo
Tha Calum an-seo > Tha e an-seo
Tha Alba fuar > Tha i fuar

Amusingly, in my personal experience, when the possessives are involved, the grammatical gender is more frequently observed:
Tha mi 'ga bualadh/'ga bhualadh etc

Does that help?

If the 'it' refers back to something earlier in the conversation text which is of feminine gender, e.g. weather (sìde, aimsir), it may well be 'i'. E.g. Tha i fuar.

I suppose you mean in a neutral situation, e.g. It's strange that.... Tha e nèonach gu bheil...
Then the default is 'e'.

This may be because the 'e' refers back to an unspoken masculine noun meaning 'a thing' or other generic term, e.g. you can also say 'Rud neònach a th'' ann...' and 'rud' is masculine, but I suspect it's just default=male, as so often , and as there is no neuter.

Perhaps one of our resident linguists will tear me to bits now - I always enjoy that.

Tapadh leibh akerbeltz & faoileag for replying.

The reason why I am asking is that I am writing a computer program that constructs simple sentences in English which you have to enter it in Gàidhlig. At the moment, I want to keep it really simple where the sentence is made up of 'Tha' plus pronoun plus adjective e.g. They are cold = Tha iad fuar. The pronouns and adjectives will be loaded from separate files and the program will construct the sentences randomly thus hopefully stopping the purpose of the program being to memorise sentences and start to use the language albeit in a very simple way.

I think for the moment I will be keeping 'It is' out of program for the time being.

You could specify the noun you're referring to with the English sentence, e.g. It is beautiful (an t-sìde), then the gender would be clearer. Or allow e as an option for i where appropriate.

If your program doesn't specify gender, you're probably OK just using "e" as a default. Not perfect, but functional.

At the moment, I am writing a simple program in Perl script for my own use. In the long term I intend to write a more comprehensive piece of the software for the KDE suite to complement the Parley program I am using. That is going to present a number of technical challenges but other non-technical challenges as well in terms of presentation Niall & GunChleoc has pointed out as I want to try and get it as immersive in the language as possible. I'll cross that bridge when I come to it.