"She will sit down in a chair."
Translation:Bidh i a' suidhe ann an cathair.
ann an (or sometimes, mostly in literature and more formal language, just an) means simple in, while in the (with the article) is always anns an, anns a’, anns na, which is sometimes shortened to san, sa, sna.
in the chair would be anns a’ chathair or sa chathair
I am also confused by the usage here compared to the earlier lesson. As noted by KatieEichenlaubW and SarahBayley we were earlier taught that "suidh sios" meant "sit down". I've tried both Am Faclair Beag and Learn Gaelic and neither appear to indicate that "sios" is required or necessary to express "sit down" as they both indicate that "down" can be implied. However that other font of knowledge, Google Translate, does translate "sit down" as "suidh sios". Is "sios" necessary to express down or does its use in "suidh sios" make the phrase an imperative?