Learn Hardcore Swedish: Det är lätt att spara pengar när vi jobbar mycket. - It is easy to save money when we work a lot.

Det does not refer to anything concrete; it just fills the subject slot.
The real “thing” being described as easy is the whole phrase att spara pengar.

So you need Det är lätt att spara pengar…, not just Lätt att spara pengar… (that would sound incomplete in normal Swedish).

What is the function of att in att spara? Is it like English to?

Yes. Att is the infinitive marker, roughly equivalent to English to before a verb.

spara = to save (infinitive)
att spara = to save

In this pattern Det är lätt att + verb, you must have att before the infinitive:

Det är lätt att spara pengar. = It is easy to save money.
Leaving out att (Det är lätt spara pengar) is ungrammatical.

Why is it spara and not sparar?

Spara is the infinitive form of the verb, used after att.

att spara = to save
jag sparar = I save / I am saving

General rule:

After att, use the infinitive: att spara, att läsa, att äta, etc.
Without att, in a normal present-tense clause, use the present form: jag sparar pengar, vi sparar pengar.

Why is pengar used without any article? When would I say pengarna or en peng?

Pengar is a plural noun that often behaves like an uncountable noun (similar to English money).

In this sentence:

spara pengar = save money in general (no article, indefinite)

Other forms:

pengarna = the money / the coins (definite plural)
- Jag har inte pengarna. = I don’t have the money.
en peng (or en slant) = a coin / a (sum of) money — used less often and with a more specific meaning.

For the general idea of money in general, Swedish usually just uses pengar with no article.

What exactly is lätt here, grammatically? Why does it come right before att?

Lätt is an adjective meaning easy. In this sentence it is a predicative adjective after är:

Det är lätt … = It is easy …

The common pattern is:

Det är + adjective + att + infinitive
- Det är svårt att förstå. = It is hard to understand.
- Det är roligt att resa. = It is fun to travel.

So lätt must come before att, because it describes how easy/difficult the action (att spara pengar) is.

Can I also say När vi jobbar mycket är det lätt att spara pengar? Is there any difference?

Yes, that word order is completely correct:

Det är lätt att spara pengar när vi jobbar mycket.
När vi jobbar mycket är det lätt att spara pengar.

Both mean the same thing.

The difference is just focus:

Starting with Det är lätt… is more neutral and focuses first on how easy it is.
Starting with När vi jobbar mycket… puts a bit more emphasis on the situation when we work a lot.

Grammatically, both are fine and natural.

Why do we use när here and not om?

När and om can both translate to when in English, but they’re used differently:

när = when / whenever (time-based, something that actually happens)
- Det är lätt att spara pengar när vi jobbar mycket.
  → When(ever) we work a lot, it is easy to save money.
om = if (condition, possibility)
- Det är lätt att spara pengar om vi jobbar mycket.
  → It is easy to save money if we work a lot.

Using om would shift the meaning to a more hypothetical “if we happen to work a lot” rather than a regular situation in time.

Could I use medan instead of när, as in Det är lätt att spara pengar medan vi jobbar mycket?

You can use medan, but the meaning changes a bit:

när vi jobbar mycket = when / whenever we work a lot (general time or situation)
medan vi jobbar mycket = while we are working a lot (overlapping in time, stressing simultaneity)

In practice:

när is more natural here, because the idea is “in periods when we work a lot, it’s easy to save money”.
medan would highlight that the saving happens at the same time as the working, which can sound slightly odd in this context.

What is the difference between jobbar and arbetar here? Are they interchangeable?

Both jobbar and arbetar mean work, and in many cases you can swap them:

när vi jobbar mycket
när vi arbetar mycket

Nuance:

jobba is more everyday/colloquial and is extremely common in speech.
arbeta is slightly more formal or used in more official contexts (contracts, formal writing, job ads).

In this sentence, jobbar sounds very natural and conversational.

Why is it mycket and not många in när vi jobbar mycket?

Both can translate to a lot / much / many, but they’re used differently:

mycket = much / a lot of for uncountable things or in a general sense
- vi jobbar mycket = we work a lot (in general, many hours)
många = many for countable things
- vi jobbar många timmar = we work many hours

Here we’re talking about the amount of work in general, so mycket is the correct choice.

How is att pronounced in this sentence? I sometimes don’t hear the t.

In everyday spoken Swedish, att (the infinitive marker) is often reduced:

In careful speech: [att] with a clear t.
In normal, fast speech it often sounds more like [a] or [å], and the t may be very weak or disappear.

So you might hear something like:

Det är lätt a spara pengar… (with a very light or missing t).

This is normal reduction in speech; in writing you always use att.