Learn Hardcore French: Je note les dépenses dans mon calendrier et dans mon agenda. - I write down expenses in my calendar and in my planner.

Un calendrier = a calendar (wall/desk calendar, or a month grid view).
Un agenda = a planner/diary (UK) or planner/daybook (US), where you schedule things day by day. Note: English “agenda” (list of topics for a meeting) is a false friend. In French that’s usually ordre du jour.

Why is it dans and not sur?

Use dans for things recorded inside a notebook/planner/app: dans mon agenda, dans mon calendrier Google.
Use sur for writing on a surface: sur mon calendrier mural (on my wall calendar). So in most “I log it in my calendar/planner” contexts, dans is the default.

Why les dépenses and not des dépenses or mes dépenses?

les dépenses: “the expenses” (a specific, known set, or all the expenses under discussion).
des dépenses: “some expenses” (unspecified subset).
mes dépenses: “my expenses” (clearly yours). Choose based on meaning: if you mean your personal spending, mes dépenses is the most natural.

Is dépenses always plural?

No. Singular is une dépense (an expense). Plural des/les dépenses is common when listing multiple items. A frequent near-synonym is plural les frais (expenses/fees), especially in administrative or reimbursable contexts.

Can I drop the repeated dans mon and say Je note les dépenses dans mon calendrier et mon agenda?

Yes. Both are correct:

Je note … dans mon calendrier et mon agenda (lighter, common).
Je note … dans mon calendrier et dans mon agenda (more explicit; fine stylistically). Do not drop the second mon: … dans mon calendrier et agenda sounds odd in French.

Could I mix prepositions and say sur mon calendrier et dans mon agenda?

Yes, if that matches reality: e.g., you physically jot amounts on a wall calendar (sur mon calendrier) and also log them in your planner (dans mon agenda).

Are calendrier and agenda masculine? What about dépense?

calendrier: masculine singular → un calendrier / mon calendrier; plural des calendriers.
agenda: masculine singular → un agenda / mon agenda; plural des agendas.
dépense: feminine → une dépense / la dépense / les dépenses.

Why mon and not mes?

Because each noun is singular: mon calendrier (one calendar) and mon agenda (one planner). Use mes only for plural nouns, e.g., mes agendas.

What does noter mean here? Could I use other verbs?

Here noter means “to write down/jot down.” Alternatives:

écrire (to write; neutral): J’écris mes dépenses…
inscrire (to enter/register in a list/form): more formal.
consigner (to record, often systematically).
enregistrer (to record/save, often digital/official).
saisir (to input/type into a system/app). Choose based on tone and medium.

Any pronunciation tips for the sentence?

Je note: the final -e in note is not pronounced; sounds like “not.”
les dépenses: no liaison; say “lé dép–” (not “léz dép–”).
mon agenda: there is a liaison; pronounce an audible “n” between mon and agenda: “mon-nagenda.”
et has no liaison and sounds like “ay.”

Could I front the place phrase, e.g., Dans mon calendrier, je note les dépenses?

Yes. Fronting Dans mon calendrier is fine for emphasis or flow. The neutral/default order is the original: subject + verb + direct object + place phrase.

Is it natural to record expenses in a calendar/agenda in French?

It’s understandable and grammatical, but many people would instead say:

Je note mes dépenses dans un tableur (in a spreadsheet),
… dans mon appli de budget, or
… dans un carnet (in a notebook). Use the medium that best fits your real habit.

Why not au calendrier or à l’agenda?

With writing/recording, French uses dans for containers or registries. à + le/la here would suggest “to the calendar” rather than “in the calendar,” so it’s not idiomatic.

Do I need a comma before et?

No. French doesn’t use a comma before et in a simple pair. Use commas only in longer lists: dans mon calendrier, mon agenda et mon carnet.