watasi ha mise de baggu wo kaimasita.

は is the topic-marking particle. It tells the listener what we are talking about—私 in this case. Key points:

• It sets 私 (“I”) as the topic, not necessarily the grammatical subject in a strict sense.
• It often contrasts or focuses on what follows; here it simply introduces who performed the action.
• が would mark the subject in a neutral or new-information context; は emphasizes “as for me…”

で indicates the location where an action takes place. Here, 店で means “at the store.” Other common location particles:

• に can indicate destination (“to the store”) or existence (“in the store”), but not the place of action.
• へ marks direction toward, also not the actual site of the action.

Japanese often omits pronouns when context is clear. You can drop 私 は if you already know who is speaking:

• 店でバッグを買いました。
  This still means “(I) bought a bag at a store.” Use 私 は when you need to clarify or contrast who did it.

バッグ is a loanword (gairaigo) from English “bag.” Katakana is used for:

• Foreign loanwords
• Onomatopoeia
• Emphasis or technical terms

買いました is the polite past form of 買う (“to buy”). Components:

• 買い- stem
• ました polite past ending
  It expresses a completed action with politeness. In casual speech, you might say 買った instead.

Japanese is typically Subject-Object-Verb (SOV):

1. Topic/Subject: 私 は
2. Location (adjunct): 店 で
3. Object: バッグ を
4. Verb: 買いました