For your first question, "this bridge" is "an drochaid seo".
The second question is a bit more complicated - you only use "tha" as the verb "to be" to describe something, not to identify something. To identify something, you use "is" in the present tense. So "tha x adjective" but "is x noun" (this gets covered in later lessons). "Is" very often gets shortened to 's and in some cases gets swallowed up in the next word altogether. This is the case with seo, sin, cò, etc. So the "seo" in our sentence is treated as containing the identifying "to be" verb - seo drochaid = 's seo drochaid = is seo drochaid. You'd say the same if you were phoning someone - seo Gordon = this is Gordon
I think (but could be wrong here, not entirely sure how copula works in Gaelic) that
is seo drochaid would not be ok, I think it would rather need to be is e seo drochaid (and definitely is e seo an drochaid for this is the bridge with definite noun), but then the copula (the is word) does weird things to syntax in Goidelic languages, and I haven’t read enough Gaelic to know it well, so I’m not sure.
I think in modern contemporary language this is a bridge would be either seo drochaid or (is e) drochaid a tha seo (roughly: it’s a bridge that this is; with a weird bi verb used with nominal predicate, instead of more regular, but afaik never used,
drochaid a tha ann seo).
edit: I corrected
is drochaid e seo to is e seo drochaid – I’ve written the first one following Irish syntax, but I’ve found the second one with indefinite predicates in a few sources (most notably Colin B.D. Mark’s Gaelic Verbs Systemised and Simplified, ch. 10, p. 173, eg. is e seo bogsa), so it seems this is the proper Sc. Gaelic syntax here.
Hi Silmeth - yes, you're right that "is seo drochaid" would not be used today in modern Gaelic - there are only a very few frozen forms where "is x y" is still current ("is mise Mairead", for example). The is or 's is just too short to be noticeable in many circumstances, so extended forms developed for clarity, like the "'s e x a th' ann an y" structure.
"an drochaid a tha seo/ an drochaid a bha seo" is another (quite common) way of saying "an drochaid seo".
Apologies if I was confusing - I was trying to explain for the OP how it came about that "seo drochaid" doesn't appear to have a verb, rather than suggest that "is seo drochaid" is interchangeable in current speech with "seo drochaid".
I’ve found several sources (some online, one more reliable – Colin B.D. Mark’s Gaelic Verbs Systemised and Simplified) claiming it should be is e seo drochaid in Sc. Gaelic (Mark gives is e seo bogsa this is a box) in the full form.
Interesting, as Irish sticks to different syntax: is droichead é seo with indefinite predicate following the copula (I think this way is older – Old Irish always had the predicate after the copula).