Jag gjorde ett dumt misstag.

Because gjorde is the simple past form of göra (to do / to make).

• Jag gjorde ett dumt misstag = I made a stupid mistake
• Jag har gjort ett dumt misstag = I have made a stupid mistake

So:

• gjorde = past tense on its own
• gjort = the form used with har in the present perfect

English learners often notice that Swedish uses göra for both do and make, so gjorde can mean either did or made, depending on the context.

Because misstag is an ett-word (a neuter noun) in Swedish.

Swedish nouns belong to one of two grammatical genders:

• en-words
• ett-words

Since it is:

• ett misstag = a mistake

the article must be ett, not en.

A few forms of the noun are:

• ett misstag = a mistake
• misstaget = the mistake
• misstag = mistakes
• misstagen = the mistakes

Notice that the plural of misstag is the same as the singular indefinite form.

Because adjectives in Swedish must agree with the noun they describe.

Since misstag is an ett-word, the adjective takes the -t form:

• en dum idé = a stupid idea
• ett dumt misstag = a stupid mistake

Basic pattern:

• dum for en-words
• dumt for ett-words
• dumma for plural and definite forms

Examples:

• en dum fråga = a stupid question
• ett dumt misstag = a stupid mistake
• dumma misstag = stupid mistakes
• det dumma misstaget = the stupid mistake

In this sentence, dumt means something like:

• stupid
• silly
• foolish

Although dum is often translated as stupid or dumb, you should not always match it word-for-word with English dumb.

In ett dumt misstag, a natural English translation is usually:

• a stupid mistake
• a silly mistake

So yes, it can correspond to dumb in some contexts, but stupid/silly/foolish is often more natural in English.

Yes, göra often means do, but it can also mean make.

Swedish does not separate do and make in the same way English does. So:

• göra ett misstag = make a mistake
• literally, it looks like do a mistake, but that is not how it is understood

This is a very common thing for English speakers to notice. You just have to learn the Swedish expression as a whole:

• göra ett misstag = to make a mistake

The word order is the normal Swedish subject–verb–object pattern:

• Jag = subject
• gjorde = verb
• ett dumt misstag = object

So:

• Jag gjorde ett dumt misstag.
• literally: I made a stupid mistake.

This is very similar to normal English word order.

Not normally in standard Swedish.

The natural expression is:

• göra ett misstag = make a mistake

The noun misstag is the standard word here. A shortened form like miss does exist in some contexts, especially informal speech or under English influence, but ett misstag is the normal and safest choice for learners.

So if you want to sound natural, use:

• Jag gjorde ett dumt misstag.

A rough learner-friendly guide is:

• Jag ≈ yahg
• gjorde ≈ YUR-deh
• ett ≈ et
• dumt ≈ dumt (with a short vowel)
• misstag ≈ MIS-stahg

A few pronunciation notes:

• In gjorde, the gj- is pronounced like y.
• The ö sound in gjorde does not exist in standard English; it is somewhat like the vowel in French deux or German schön.
• misstag has the stress on the first syllable: MIS-stag.

A more natural connected pronunciation may sound a bit smoother and less separate than the spelling suggests.

Usually no, not in a normal full sentence.

Swedish normally requires the subject to be stated:

• Jag gjorde ett dumt misstag.

Unlike some languages, Swedish does not usually drop subject pronouns in ordinary sentences.

You might omit words in very informal speech, notes, or diary-style writing, but as a learner you should keep Jag there.

Yes, ett dumt misstag is indefinite:

• ett = a
• ett dumt misstag = a stupid mistake

To say the stupid mistake, Swedish usually uses both a separate definite marker and a definite ending on the noun:

• det dumma misstaget

Compare:

• ett dumt misstag = a stupid mistake
• det dumma misstaget = the stupid mistake

Notice that the adjective changes too:

• dumt → dumma
• misstag → misstaget

You would use the present tense of göra, which is gör:

• Jag gör ett dumt misstag.

Grammatically, that means I make / am making a stupid mistake, but in real life it is less common than the past tense, because people often talk about a mistake after it happened.

Other useful versions:

• Jag gjorde ett dumt misstag. = I made a stupid mistake.
• Jag har gjort ett dumt misstag. = I have made a stupid mistake.
• Jag gör ofta dumma misstag. = I often make stupid mistakes.

Because misstag is one of those Swedish nouns whose indefinite plural is the same as the singular.

So you get:

• ett misstag = a mistake
• flera misstag = several mistakes

Then the definite plural is:

• misstagen = the mistakes

This can feel strange to English speakers at first, but it is completely normal in Swedish. Many neuter nouns behave in similar ways.

Yes, but it sounds more formal or literary.

• göra ett misstag = the normal everyday expression
• begå ett misstag / past begick = a more formal way to say commit a mistake / make an error

So:

• Jag gjorde ett dumt misstag = the most natural everyday version
• Jag begick ett dumt misstag = understandable, but more formal or dramatic

For everyday Swedish, gjorde is the better choice.