Pystyn tekemään tämän tänään.

It is built into pystyn.

In Finnish, the verb often shows the subject, so you do not need to say the pronoun unless you want extra emphasis.

• pystyn = I am able / I can manage
• the ending -n marks first person singular: I

So the full sentence could also be:

• Minä pystyn tekemään tämän tänään.

But minä is usually left out unless you want to stress I specifically.

Pystyn is the 1st person singular form of pystyä.

It means something like:

• I am able to
• I can manage to
• I’m capable of

Compared with English can, pystyä often emphasizes actual ability or whether something is possible for you in practice.

So Pystyn tekemään tämän has a sense like:

• I’m able to do this
• I can manage to do this

Because pystyä normally takes the third infinitive in the illative, which has the ending -maan / -mään.

So:

• tehdä = the basic infinitive, to do
• tekemään = the form used after verbs like pystyä

This pattern is very common:

• pystyn tekemään = I am able to do
• en pysty tekemään = I am not able to do

How it is formed:

• tehdä → stem tekemä- → tekemään

So this is not random: it is the normal structure after pystyä.

Because tämän is the object form here.

The basic dictionary form is:

• tämä = this

But in the sentence, this is what is being done, so it appears as the object:

• tämän = this as the object in a completed/whole sense

In Pystyn tekemään tämän tänään, the idea is that the whole thing can be done today, so Finnish uses the total object form.

A useful contrast:

• Teen tämän tänään. = I’ll do this today.
• Teen tätä tänään. = I’ll be working on this today.

So:

• tämän suggests the action is seen as complete or as a whole
• tätä would suggest something ongoing, partial, or incomplete

For a learner, the most useful answer is: it is the object form used for a total object.

Historically and grammatically, forms like tämän, sen, tuon are often described as accusative in object use, even though they look like genitive forms.

So in practice:

• tämän here is the correct object form after do this
• you do not need to worry too much about the label at first
• the important thing is that Finnish uses tämän when the object is treated as whole/complete

Tänään means today, and in this sentence it works as a time adverb.

It tells you when the action can happen.

So the structure is roughly:

• Pystyn = I can / I’m able
• tekemään = to do
• tämän = this
• tänään = today

Unlike many nouns, tänään is just used here as a fixed time expression.

Yes, you can, and it is very natural.

But there is a small nuance:

• Pystyn tekemään tämän tänään = emphasizes ability / capability / managing
• Voin tehdä tämän tänään = can mean I can do this today, but may sound a bit broader, depending on context

Very roughly:

• pystyä = be able, be capable, manage
• voida = can, may, be able to

Sometimes voida can also sound like it is possible / I am free to / I am allowed to, depending on context.
Pystyä more clearly points to actual ability.

So if you want to stress I am capable of getting this done today, pystyn is a very good choice.

Yes. Finnish word order is more flexible than English word order.

The neutral version here is:

• Pystyn tekemään tämän tänään.

But you could also say:

• Tänään pystyn tekemään tämän.
• Pystyn tänään tekemään tämän.

These all mean roughly the same thing, but the emphasis changes a little:

• starting with tänään highlights today
• keeping tänään at the end is a neutral way to add the time information

So Finnish word order is often used for focus and emphasis, not just grammar.

Not quite.

It means I am able to do it today or I can do it today. It expresses ability/possibility, not a firm promise.

So it is different from:

• Teen tämän tänään. = I will do this today / I’m doing this today

Compare:

• Pystyn tekemään tämän tänään. = I am able to do this today
• Teen tämän tänään. = I will do this today

So the sentence says the speaker has the ability or opportunity, not necessarily that the action is already decided.

Because Finnish usually expresses that idea with an infinitive form, not with a separate word like English to.

In English:

• to do

In Finnish:

• tehdä or, after pystyä, tekemään

So the meaning of English to is built into the verb form itself. Finnish does not need a separate little word before the verb.