Dé bhur mbeatha, a cháirde!

Tá feabhar agam le hiarraidh oraibh.

Tá leabhar léinn agam, agus is é teideal atá air ná "Introduction to Studies in Modern Irish". Is éachtach é chun foghlamtha.
Tá mórán cleachtaí aistrithe ann, Béarla ar fuaid an leabhair agus Gaelainn bhinn 'na dheireadh.
Tar éis Gaelainn do chur ar an mBéarla sin dom féin, cuirim i gcomparáid le Gaelainn atá i ndeireadh an leabhair í.
Ar maidin inniu, bhíos ag foghlaim na Gaelainne, ag déanamh na gcléachtaí sin (aistriú), mar is gnáth dom. Ach féach, do fuaras ná fuil cuid díobh le fáil ar bith.

Is é aistriú na n-abairtí sin an fabhar.

Exercise C (pp. 132-133)
4. Don’t touch that knife for fear you might cut your finger with it.
5. If you have a pencil you may as well write your lesson with it.
Exercise CI (p. 133)
1. If you’ve nothing to do inside you’d better go out into the air.
2. I’m afraid there’s more misfortune to come on this country yet.
3. I never saw two people so like each other as Tomás and his father.

Do scríobhas-sa:
C-4. Ná béir ar an scian so le h-eagla go ngearrair do mhéar léi.
C-5. Má tá peann luaidhe agat, ní miste dhuit do cheacht a scríobh leis.
CI-1. Mura bhfuil pioc le déanamh agat istigh, is fearra dhuit dul amach fé'n spéir.
CI-2. Is baoghalach liom go bhfuil tuilleadh donais le teacht ar an dtír seo.
CI-3. Ní fhaca riamh an bheirt atá chómh deallrathach le chéile agus atá Tomás agus a athair.

Cad a scríobhfadh sibh-se? Abraidh liom é, má's é bhur dtoil é.

Is mise le meas
Gréasaghas

[Please excuse my poor Irish. I would like to request help regarding exercises 100 (4-5) 101 (1-3) pp. 132-133 (both about the use of 'le') from Introduction to Studies in Modern Irish by father Gerald O'Nolan. The key is partially missing, viz. the Irish translations of the five sentences above. I gave it a go and would be very grateful if you could correct my Irish.]

C-4 maybe: ná bain leis an sciain
C1-1 maybe: b'fhearra dhuit? Or: is é rud is fearra dhuit.
CI-2: is baolach or is baolach liom

The book is at https://archive.org/details/introductio ... dern+Irish, but as you have already found out the key doesn't match the exercises entirely. It's as if some of the sentences were changed, but they forgot to change the key.

If you look at the answr to CV no. 5 it says Ní fheaca riamh aon bheirt ba mhó cion ar a chéile ná iad. Clearly the author changed the sentence at the last minute. Maybe: ní fheaca riamh AON bheirt ba mhó cosúlacht lena chéile ná iad? (edited from ar a chéile to lena chéile)

Go raibh maith agat!

C-4: Níl an briathar 'bainim le' i gluais an leabhair so ach is dóigh liom, mhuise, go bhfuil an ceart agat.
'beirim ar' is of course 'seize' (etc.), but this is the closest equivalent I've found in the book.

CI-3: aon bheirt: use of aon with the plural always trips me up.
Would this work (to use deallrathach/deallramtach/dealraitheach that is given in the examples in Lesson CIV)?
Ní fheaca riamh aon bheirt chómh deallrathach lena chéile le Tomás agus a athair.

I must admit that those two 'le's look a bit weird.
Also: are 'lena chéile' and 'le chéile' interchangeable here?

Corpas Stairiúil gave me:
É sin agus Cnoc Tuirc cois Léin-Locha, táid siad deallrathach len a chéile ar an nós san. (Tíreóluidhe ar Éirinn - Tóibín, Seán)
Tá siad chómh deallrathach le chéile is dá mbainfeá an ceann de dhuine aca is é chur ar an nduine eile. (Mac Cárthaigh Mór - Ó Séaghdha, Pádraig)

Yes. I've been using a different scan available at https://archive.org/details/cu31924026508022, but the one you linked seems to be an exact reprint (with the dates strangely missing). The key has been quite workable so far. It's only these two exercises that don't have complete answers.

I don't see a difference between le chéile and lena chéile. Maybe others can comment?

When discrepancies arise, considering context and the logical structure of the language can often help you deduce the correct answers.