وقتی به خانه میرسم، در را قفل میکنم.

Because Persian often drops subject pronouns when the verb ending already shows who the subject is.

• می‌رسم = I arrive / I get
• می‌کنم = I do / I lock

The ending -م tells you the subject is I, so من is usually unnecessary unless you want emphasis:

• وقتی من به خانه می‌رسم... = When I get home...
  This sounds more emphatic than the version without من.

وقتی means when here.

It introduces a time clause:

• وقتی به خانه می‌رسم = when I get home / when I arrive home

It can also sometimes mean whenever, depending on context. In this sentence, it most naturally gives a habitual meaning:

• Whenever/When I get home, I lock the door.

به usually means to.

So:

• به خانه = to home / home

With verbs of motion like رسیدن (to arrive, to reach), Persian often uses به before the destination:

• به خانه رسیدن = to arrive home
• به مدرسه رسیدن = to arrive at school

So وقتی به خانه می‌رسم literally means when I arrive to home, but in natural English we say when I get home or when I arrive home.

خانه can mean both house and home, depending on context.

In the phrase به خانه رسیدن, it usually means to get home. Persian does not always need a possessive like my in expressions like this.

So:

• به خانه می‌رسم = I get home
• not necessarily I arrive at the house

If you wanted to be more explicit, you could say:

• به خانه‌ام می‌رسم = I arrive at my home

But in everyday Persian, به خانه is very normal for home.

می‌رسم comes from the verb رسیدن.

That verb means:

• to reach
• to arrive

So:

• می‌رسم = I arrive / I reach
• به خانه می‌رسم = I arrive home / I get home

Its structure is:

• می- = present/imperfective marker
• رس = present stem
• -م = I

So می‌رسم literally means I am arriving / I arrive, depending on context.

Because Persian often uses the simple present for habitual or repeated actions, just like English does in sentences such as:

• When I get home, I lock the door.

This sentence describes a regular pattern, not one specific event:

• وقتی به خانه می‌رسم، در را قفل می‌کنم.
• When I get home, I lock the door.

So the present tense here has a habitual meaning.

If you were talking about a one-time future event, Persian might use a different structure, for example:

• وقتی به خانه برسم، در را قفل می‌کنم.
• When I get home, I’ll lock the door.

را is the direct object marker.

It usually marks a specific direct object. Here, the object is در (the door), and را shows that it is the thing being locked.

So:

• در را قفل می‌کنم
• literally: the door [object marker] lock I-do
• natural English: I lock the door

A very important point: Persian has no word exactly like English the, but را often goes together with a definite, specific object.

So در را is best understood as the door here, not just a door.

Persian usually puts the verb at the end of the clause.

So the order is often:

• subject
• object
• verb

In this sentence, the second clause is:

• در را قفل می‌کنم

That breaks down as:

• در را = the door
• قفل می‌کنم = I lock

So Persian says something more like:

• the door, I lock

This is normal Persian word order.

It is a compound verb.

• قفل = lock
• کردن = to do

Together:

• قفل کردن = to lock

So:

• قفل می‌کنم = I lock
• literally: I do lock

This kind of compound verb is extremely common in Persian. Many actions are expressed with a noun or adjective plus a light verb like کردن.

Yes, بستن / می‌بندم means to close.

But قفل کردن specifically means to lock.

So there is an important difference:

• در را می‌بندم = I close the door
• در را قفل می‌کنم = I lock the door

A door can be closed without being locked. This sentence clearly means lock.

In standard modern Persian spelling, it is better written with a half-space:

• وقتی به خانه می‌رسم، در را قفل می‌کنم.

So instead of:

• میرسم
• میکنم

you will often see:

• می‌رسم
• می‌کنم

Both are understandable, and many people write without the half-space in casual typing, but می‌رسم and می‌کنم are the more standard forms.

A natural pronunciation is approximately:

• vaqti be khâne miresam, dar râ qofl mikonam

A few notes:

• وقتی ≈ vaqti
• خانه ≈ khâne
• می‌رسم ≈ miresam
• در را ≈ dar râ
• قفل ≈ qofl
• می‌کنم ≈ mikonam

Depending on accent and transliteration style, you may also see vaghti instead of vaqti.

It can feel like either, but in most contexts it has a habitual sense:

• When / Whenever I get home, I lock the door.

Because both verbs are in the present tense, the sentence usually describes something that regularly happens.

If you wanted a very specific one-time future meaning, Persian would often change the first verb, for example:

• وقتی به خانه برسم، در را قفل می‌کنم.
• When I get home, I’ll lock the door.