Cuando compro algo, lo tacho enseguida de la lista de la compra.

Because this sentence describes a habit or something the speaker usually does.

In Spanish, when cuando means whenever in a repeated situation, the present tense is normal:

• Cuando compro algo, lo tacho... = Whenever I buy something, I cross it off...

It is not talking about one single event in the future, but about a routine.

Spanish often omits subject pronouns when they are not needed. The verb ending already shows who the subject is:

• compro = I buy
• tacho = I cross off

So yo is understood. You could say Cuando yo compro algo, lo tacho..., but it sounds less natural unless you want emphasis.

Here tachar means to cross out or to cross off.

In the context of a shopping list, tachar algo de la lista means you mark an item as already bought, usually by drawing a line through it.

So:

• tachar una palabra = cross out a word
• tachar algo de la lista = cross something off the list

Because lo refers to algo, not to la lista.

That is the key point:

• algo is the thing that was bought
• la lista is the list

So the sentence means:

• When I buy something, I cross it off the shopping list

The direct object pronoun replaces algo, and Spanish uses lo here.

Even though lista is feminine, that is not the thing being replaced by the pronoun.

Because tachar needs a direct object: you cross off something.

In the first part, that something is introduced as algo. In the second part, Spanish repeats it with the object pronoun:

• Cuando compro algo, lo tacho...

Without lo, the sentence would feel incomplete, because you would be saying I cross off immediately from the list, without clearly stating what you cross off.

Enseguida means right away, straight away, or immediately/very soon depending on context.

In this sentence, it means the speaker does it as soon as possible after buying the item:

• lo tacho enseguida = I cross it off straight away

In Spain, enseguida is very common in everyday speech.

Because the normal Spanish expression is tachar algo de una lista = to cross something off a list.

English uses off, and Spanish often uses de in this structure.

Compare:

• tachar algo de la lista = cross something off the list

Using en la lista would sound unnatural here, because the idea is not location (on the list) but removal from the active list.

In Spain, la lista de la compra is a very common way to say shopping list.

Literally, it is the list of the shopping/purchase. This is a fixed everyday expression.

You may also hear:

• la lista de compras in some other varieties of Spanish
• la lista del súper informally

But in Spain, la lista de la compra is especially natural.

Because in Spain Spanish, la compra often means the shopping as a general activity or errand.

So:

• hacer la compra = do the shopping
• la lista de la compra = the shopping list

It is not about counting individual purchases, so the singular is normal.

Here algo means something.

So:

• Cuando compro algo... = When/Whenever I buy something...

It refers to any item, but grammatically it is still something, not anything.

Yes, but the nuance changes a little.

• Cuando compro algo... = Whenever I buy something... / When I buy something...
• En cuanto compro algo... = As soon as I buy something...

In your sentence, cuando works very well for a habitual action.
If you use en cuanto, it stresses the immediate sequence even more.

Yes, enseguida is fairly flexible.

Your sentence has:

• lo tacho enseguida

You could also hear:

• enseguida lo tacho
• lo enseguida tacho → not natural

The most natural positions are usually:

• after the verb: lo tacho enseguida
• before the whole verb phrase for emphasis: enseguida lo tacho

Yes, it sounds very natural in Spain, especially because of expressions like:

• enseguida
• la lista de la compra

The grammar itself is standard Spanish, but la lista de la compra is particularly associated with Spain. In many parts of Latin America, people might prefer la lista del supermercado or la lista de compras, depending on the country.