watashi wa kazoku ni kansha no kimochi o messeeji de tsutaemashita.

I conveyed my feelings of gratitude to my family by message.

Breakdown of watashi wa kazoku ni kansha no kimochi o messeeji de tsutaemashita.

• は: topic particle
• 私わたし: I
• を: direct object particle
• の: possessive case particle

Questions & Answers about watashi wa kazoku ni kansha no kimochi o messeeji de tsutaemashita.

Why is the particle は used after 私 here?

は marks the topic of the sentence.

• 私 = I / me
• 私＋は = As for me / Speaking about me

So the basic structure is:

• 私は = As for me,
• 家族に = to my family,
• 感謝の気持ちを = feelings of gratitude (as the thing),
• メッセージで = by message,
• 伝えました = (I) conveyed (them).

In many contexts, natural Japanese would actually omit 私 and just say:

• 家族に感謝の気持ちをメッセージで伝えました。

Because it is obvious from context who the subject is. は here does not mean “subject marker” in a strict grammatical sense; it mainly introduces what the sentence is about.

Why is 家族 marked with に and not を?

に here marks the recipient / target of the action.

• 家族に = to my family / toward my family

In this sentence:

• The thing being conveyed is 感謝の気持ち → marked with を (direct object).
• The person receiving that thing is 家族 → marked with に (indirect object / target).

So:

• 感謝の気持ちを伝える = to convey one’s feelings of gratitude
• 家族に感謝の気持ちを伝える = to convey one’s feelings of gratitude to one’s family

If you used 家族を, it would suggest that 家族 is the thing being conveyed (which makes no sense here).