。
Mother and I were in the garden.
Literal
Mother [and-と] I [topic-は] garden [in-に] was (polite).
The particle と joining 母 and 私 is the exhaustive 'and' for enumerated items — the whole conjunction 母と私 becomes the topic marked by は. いました is the polite past of いる (for animate existence); 庭 takes に as the location of being.