Epistemic Modality or Do I HAVE to read this?

Don't worry, that's just one of those lovely linguistic terms that you get when you look up modal verbs.

Expressions of obligation and necessity tend to be tricky in any language, so you will be glad to hear that Gaelic is no different!  And before we start, I would like you to think about the many ways in which we can express necessity to do something in English: must, have to, be compelled, be forced, be obliged, be required, got to, ought, should ... all those words we hated as children.

So how is Gaelic tricky?  Well, because it has about 5 basic expressions which can all be translated as 'must' into English.  They aren't all the same however, some differ in meaning slightly, some in focus and some in use.  These basic expressions are (all given in the 1st person): feumaidh mi, bidh agam ri, thig orm, is fheudar dhomh and is éiginn dhomh.

Here we've already encountered our first hurdle: which tense to give them in because as we will see, you can't use them indiscriminately in just any tense.

For a change, you'll get the solution first and the explanantions afterwards:

So what's the catch?

Let's start with feumaidh.  This word, which interestingly comes from the Old Irish word feidm 'need, use' has two basic meanings and/or usages.  It first of all can mean that something is required, for example feumaidh mi airgead (a chum na siùrsaich a phàigheadh) 'I need money (to pay the hooker)'.  This in a way is the 'original' use of the word and is close to the modern Irish usage eg tá mé i bhfeidhm gabhair 'I am in need of a goat' (don't ask).  This word later extended its meaning to include 'must, have to' in Gaelic.  For example feumaidh mi an t-siùrsach a phàigheadh 'I have to pay the hooker'.

Tragically, it's one of our defective verbs (which means it's a verb which does not have forms for all the tenses) so we'd be a bit stuck if this was the only verb meaning 'must'.

Notice (we'll talk a bit more about this later) that this verb is strictly a future form: feumaidh mi, a dh'fheumas mi, chan fheum mi ... so there is a distinct flavour of things which are or can happen yet to this verb (yes, there are conditional forms too but we'll get to that later).

Notice also that in comparison to is fheudar dhomh and tha agam ri, feumaidh has a slightly more intentional flavour eg feumaidh mi an leabhar seo a leughadh 'I have to read this book (because I heard it's really good) wheres tha agam ris an leabhar seo a leughadh 'I have to read this book (because I have to write an essay on it by tomorrow)'.

In comes Old Irish fétir.  Originally this meant 'possible' and still does in Irish eg is féidir liom ranníocaíochtaí deonacha a íoc 'I can pay voluntary contributions'.  In Gaelic though, this word has undergone a partial shift to supply past and present tense forms for feumaidh (which tends to be present/future).  For example b' fheudar dhomh an t-siùrsaich a phàigheadh 'I had to pay the hooker'. 

The first example would have to be expressed in a roundabout way by the way: bha feum agam air airgead a chum na siùrsaich a phàigheadh ... 'I needed money to pay the hooker'.  Sooo ... is fheudar and b' fheudar mean pretty much the same thing as feumaidh in the sense of 'must' except that this one has a present/past focus.

Confusingly though, this verb retains its original meaning in a number of expressions and usages, especially with negatives: rud nach b' fheudar 'a thing that wasn't possible'; chan fheudar nach dig mi 'I will certainly come (=It's not possible for me not to come)'.

Next in line is bidh agam ri.  This is perhaps the most straightforward of all the 'must' expressions as it expresses an obligation of 'must', plain and simply and in every tense availavble ie bidh agam ri, tha agam ri, bha agam ri, bhiodh agam ri ... Tha agam ri siùrsach a phàigheadh 'I have to pay a hooker'.

Thig orm now is interesting because it also gets translated as 'have to, must' but has a different focus.  Whereas with the others the focus can be either internal or external, thàinig orm an leabhar seo a leughadh 'I had to read this book' has a strong element of some external factor making you read it.  It's perhaps better translated as 'I was compelled to read the book'.

The last one is quite similar but simply with more urgency: b' éiginn dhomh falbh oir bha mo thaigh air theine 'I had to(was forced to leave because my house was on fire'.  Considering that éiginn means something between distress and necessity, that is not really surprising.  There's also the expression is éiginn dhomh 'I am forced to'.

Here's a full paradigm of feum:

Note also that although a root form feum exists, this is not used for issuing commands, rather feumaidh is used eg feumaidh tu seo a dhèanamh 'you must do this'.

Beyond these expressions there are some more, as in English but not with the basic meaning of 'have to, must' (the list is not exhaustive):