Marcamos las fechas importantes en el calendario y en el sitio web familiar.

It’s ambiguous in writing. For -ar verbs, the first-person plural is spelled the same in the simple present and the preterite: marcamos.

• Present/habit: You’d clarify with adverbs like siempre, normalmente, or with context: “We (usually) mark…”
• Completed past: You’d clarify with time markers like ayer, la semana pasada, ya: “We marked… yesterday.”
• Right-now action: Use the progressive: Estamos marcando…
• Habitual past: Marcábamos… Note: In Latin America, the preterite (e.g., marcamos ayer) is preferred over the present perfect for completed past actions.

Both are possible, but they mean slightly different things:

• Marcamos las fechas importantes… implies a specific, known set (e.g., the family’s important dates).
• Marcamos fechas importantes… is more general/indefinite (“we mark important dates” as a practice). Choose las if the set is known or previously mentioned; omit it for a general statement.

All are correct:

• El sitio web familiar is fine and common, but can be read as “family-friendly.”
• El sitio web de la familia explicitly says “the family’s website.”
• Nuestro sitio web is the most direct if the speakers are the family. Choose based on how explicit you want to be about possession.

Yes, depending on nuance:

• Anotar / apuntar: “to write down, note.” More about recording text. Example: Anotamos las fechas…
• Señalar: “to indicate/flag.” Slightly more general; can be marking in a list or pointing out.
• Registrar: “to register/log.” More formal/administrative. Marcar suggests physically marking/highlighting or otherwise tagging them as important.

Use the inclusive imperative with the present subjunctive:

• Marquemos las fechas importantes… You can also use:
• Vamos a marcar las fechas importantes… (very common for suggestions). Note: Marcamos… (indicative) is not the standard way to say “let’s…,” though ¿Marcamos…? can work as a casual suggestion in a question.

Yes. To keep the hard “k” sound before e/i, c changes to qu:

• Yo (preterite): marqué (not “marcé”).
• Imperative/subjunctive: marque, marquemos, marquen. No spelling change is needed before a/o/u: marcamos, marcabas, etc.

It depends:

• Calendario = a calendar (wall calendar, app calendar view).
• Agenda = a planner/diary (the thing you carry to schedule tasks/appointments). If you mean a planner, agenda works. If you mean the calendar itself, stick with calendario.