The Preposition "U" (At Someone's Place)
In Polish, to say that you are "at someone's house" or visiting a professional's workplace, you use the spatial preposition u followed by the Genitive case.
When using pronouns with u, you must use their Genitive forms. For example, ja becomes mnie, on becomes niego, and ona becomes niej.
If the person whose place you are at is the subject of the sentence (e.g., "I am working at my own place"), use the reflexive pronoun siebie.
This structure is very common when talking about staying over. The verb nocować means "to spend the night."
Appointments and Errands
You will frequently use u when running errands at a business named after the professional who works there, such as a doctor, dentist, or hairdresser. Remember to put the profession into the Genitive case.
Most masculine professions will end in -a or -y in the Genitive (e.g., fryzjer becomes fryzjera, okulista becomes okulisty).
If the profession is a feminine noun that declines like an adjective, such as krawcowa (dressmaker), its Genitive ending is -ej.
Visiting Family and the Verb "Bywać"
The same rules apply when visiting family members. The noun teść (father-in-law) becomes teścia, and teściowa (mother-in-law, which declines like an adjective) becomes teściowej.
To say that you are somewhere frequently or visit often, use the habitual verb bywać (to visit frequently / to tend to be).
Picking Things Up
While running errands, you often need to pick something (or someone) up. Polish uses the verbs odebrać (perfective, for a completed action) and odbierać (imperfective, for an ongoing or habitual action) for this purpose.
As a helpful bonus: odbierać is also the verb used for "picking up" or answering a phone call!