było

it was (third-person singular neuter past of być)

sobie

1. dative/locative singular of się
2. (colloquial) dative/locative singular of się
3. dative/locative singular of się

miasteczko

diminutive of miastko

nad

1. denotes location; above, over
2. denotes location, particularly in relation to a body of water; on, by
3. denotes motion; to above, to over
4. denotes movement, particularly in relation to a body of water; to
5. denotes position in comparison; than; over
6. over (indicates relative status, authority or power)
7. denotes highest degree of intensity of an action or state;
8. denotes subject of activities; on
9. with a reduplicated noun, expresses superlative nature; the greatest of all, of

rzeczką

instrumental singular of rzeczka