When using past tense particles i read that you change the ending if the stem ends in ㅂ. ex. 춥다 = 추웠습니다. However, the lesson i am on now is using 입다 and leaving it as 입었습니다. Is there a reason for this or is it just an irregularity?

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technically 춥다 is called ㅂ-irregular (ㅂ 불규칙) while 입다 is regular, but i think there are far more ㅂ-irregular verbs/adjectives than the regular ones.

you can’t tell if one is regular or not by just looking at it(*) and you just have to memorize which one is which. in particular, 굽다 can be in both camps:

고기를 구웠다 (irregular when meaning to roast sth)
길이 굽었다 (regular when meaning for roads to wind)

*: actually you can, for some of them: adjectives (descriptive verbs) ending with -답다, -롭다, and -스럽다 are all irregular.

edit: regular ones are always regular and ditto for irregular ones. that is, 입다 is always 입어서 and never 이워서, and likewise there’s never a 춥어서. you can regard 굽다 above as two different (albeit identical looking) words.

edit2: 곱다 can be in both camps too.
손이 고와서 (irregular when meaning to be beautiful)
손이 곱아서 (regular when meaning for hands/feet to be numb with cold)

edit3: among irregular ones, only 곱다 and 돕다 have their ㅂ changed to 오; all others have theirs changed to 우.