Agradezco haber anotado la fecha en mi agenda antes de salir.

Spanish often drops subject pronouns when the verb form already makes the subject clear. Agradezco already means I am grateful / I appreciate, so yo is unnecessary unless you want emphasis or contrast.

• Agradezco haber anotado... = neutral
• Yo agradezco haber anotado... = more emphatic, like I am grateful

Agradezco is the 1st person singular present of agradecer.

Here it means something like:

• I’m grateful for...
• I appreciate...

In this sentence, it expresses gratitude or appreciation about a past action.

Haber anotado is the perfect infinitive. It refers to an action that happened before the feeling expressed by the main verb.

So:

• agradezco anotar would sound like appreciating the act in a general or ongoing sense
• agradezco haber anotado means I’m grateful that I wrote it down earlier

It points to a completed past action.

It is made up of:

• haber = infinitive of the auxiliary verb
• anotado = past participle of anotar

Together, haber + past participle forms the perfect infinitive.

Examples:

• haber comido = to have eaten
• haber llegado = to have arrived
• haber anotado = to have written down / noted

Because after agradezco, Spanish does not normally use que + indicative to talk about your own completed action in this way.

Instead, Spanish prefers the infinitive construction:

• Agradezco haber anotado...

If you use que, the structure usually changes and often takes the subjunctive, especially when another person is involved:

• Agradezco que hayas anotado la fecha... = I’m grateful that you wrote down the date...

So the infinitive is the natural choice here.

The most natural interpretation is that the same person who says agradezco also did the writing down.

So:

• I appreciate having written down the date...

That is a common pattern in Spanish: when the subject stays the same, an infinitive structure is often used.

Yes. If the person who wrote the date is different, Spanish usually uses que + subjunctive:

• Agradezco que hayas anotado la fecha en mi agenda antes de salir.
  = I appreciate that you wrote the date in my diary/planner before leaving.

So:

• Agradezco haber anotado... = I wrote it down
• Agradezco que hayas anotado... = you wrote it down

Anotar means to note down, to write down, or to jot down.

It is similar to escribir, but more specific:

• escribir = to write
• anotar = to write something down for future reference

So anotar la fecha en mi agenda suggests recording the date so you do not forget it.

Not usually. In Spanish, agenda often means:

• a planner
• a diary
• an appointment book
• a personal schedule book

So en mi agenda means something like in my planner or in my diary, not in my agenda in the English meeting-topic sense.

Because the date is being written inside or in the planner, so Spanish uses en.

• anotar algo en una agenda
• escribir algo en un cuaderno

Using a would not fit this idea.

After antes de, Spanish uses the infinitive when the subject is the same or when the subject is understood in a general way.

So:

• antes de salir = before leaving / before I left

If the subject changes, Spanish uses antes de que + subjunctive:

• antes de que salieras = before you left
• antes de que saliera = before he/she left

Here, antes de salir is correct because it refers naturally to the same person.

It can, but in this sentence it more broadly means to leave.

Depending on context, salir could mean:

• to leave the house
• to go out
• to head out
• to depart

So antes de salir is best understood as before leaving.

Spanish often uses the definite article where English might use the, a, or sometimes nothing, depending on context.

Here la fecha suggests a specific date already known from the situation:

• the date of an appointment
• the date of an event
• the date we were talking about

So it sounds natural as the date.

Yes, it is grammatical and understandable, though agradezco can sound a bit formal or reflective depending on context.

More everyday alternatives might be:

• Me alegro de haber anotado la fecha... = I’m glad I wrote down the date...
• Menos mal que anoté la fecha... = Good thing I wrote down the date...

But Agradezco haber anotado la fecha en mi agenda antes de salir is still correct and natural enough, especially in a more careful or thoughtful style.

Yes. That is a very useful literal translation.

• haber anotado = to have written down
• agradezco haber anotado... = I’m grateful to have written down...

Even if the best English translation may be slightly different in natural speech, that literal version helps you understand the grammar.