Det skulle gerne blive en lille overraskelsesfest, så du må ikke fortælle hende noget om kagen.

In this sentence, skulle is not really a past-time form. Danish often uses the past-form shape of a modal verb to sound softer, less direct, or more tentative.

So det skulle gerne blive ... means something like it should hopefully turn into ... or it is supposed to hopefully become ..., not it was supposed to in a past sense.

Skulle gerne is an idiomatic combination. It expresses an expectation or hope that something will happen in a certain way.

Here it suggests that the speaker hopes the plan will work out: it should hopefully become a little surprise party. The word gerne does not mean gladly here in the usual English sense; it helps create that hopeful, polite tone.

Here det works as a kind of dummy subject, similar to English it in sentences like It will be fun or It became a problem.

It does not point to one clear concrete noun. Instead, it refers loosely to the whole situation or plan.

Blive means become or turn into, while være means be.

So det skulle gerne blive en lille overraskelsesfest suggests that the event is going to develop into a surprise party. If you used være, it would sound more like you were simply stating what it is, rather than what it is going to become.

Literally, lille means small. But in everyday Danish, it can also make something sound modest, friendly, or affectionate.

So en lille overraskelsesfest can mean a physically small party, but it can also just mean a nice little surprise party or a modest surprise party.

Danish normally writes compound nouns as one word. So overraskelsesfest is built from overraskelse + fest.

The -s- in the middle is a linking sound that often appears in compounds. You do not always have to predict it from English, so compounds like this are often best learned as whole words.

Here så means so, therefore, or in that case.

It connects the two main ideas: because it is meant to be a surprise party, the listener must not tell her anything about the cake.

That is because så here is a coordinating conjunction, like English so. After a coordinating conjunction, Danish keeps normal main-clause word order: subject + verb.

So you get så du må ikke fortælle .... If you used an adverb like derfor, then inversion would happen: Derfor må du ikke fortælle ...

Må ikke means must not or are not allowed to. It expresses prohibition or lack of permission.

So this is stronger than just saying someone should preferably not do it. It means the person is not supposed to reveal anything. Learners often confuse this with kan ikke, which means cannot, and with skal ikke, which can mean something more like do not have to or are not supposed to depending on context.

Noget om means something about, and in this kind of negative sentence it often feels like anything about.

So du må ikke fortælle hende noget om kagen means you must not tell her anything at all about the cake. If the sentence were just fortælle hende om kagen, it would still make sense, but noget adds the idea of any information whatsoever.

Because hende is the indirect object, the person receiving the information, and noget is the direct object, the thing being told.

In Danish, with verbs like fortælle, it is very common to put the indirect object before the direct object: fortælle hende noget. This is similar to English tell her something.

Kagen is the definite form, meaning the cake. It shows that the speaker and listener both know which cake is being talked about.

So this is not just any cake in general; it is a specific cake connected to the planned surprise.