Я сразу догадался, что это не тот адрес.

Because the verb is догадаться, not догадать in this meaning.

• догадаться means to guess, to realize, or to figure out
• the -ся is part of the verb’s dictionary form here

So:

• Я догадался = I guessed / I realized
• Я догадал would sound wrong in this meaning

In other words, you should learn догадаться as a whole verb.

The -ся is the reflexive marker, but in many Russian verbs it does not translate as a literal English myself.

With догадаться, the whole verb means something like:

• to figure out
• to catch on
• to realize
• to guess correctly

So the -ся is just part of the normal verb form you need to memorize. It is not something you would usually translate word-for-word in English.

This is a question of aspect.

• догадаться is perfective
• догадываться is imperfective

Here, догадался shows a completed moment of realization: I immediately realized / figured it out.

That fits well with сразу because the sentence describes a single finished moment when the speaker understood the situation.

Compare:

• Я сразу догадался = I realized immediately / I figured it out right away
• Я догадывался = I had a suspicion / I was guessing / I suspected

So догадался is used because the speaker is talking about one completed insight.

That is the past tense masculine singular form.

Russian past tense agrees with gender and number:

• догадался = masculine singular
• догадалась = feminine singular
• догадалось = neuter singular
• догадались = plural

So Я сразу догадался tells you the speaker is male, or at least is using the masculine form.

If the speaker were female, it would be:

• Я сразу догадалась, что это не тот адрес.

Сразу means right away, immediately, or at once.

In this sentence it modifies the verb догадался:

• Я сразу догадался = I realized immediately / I figured it out right away

It gives the sentence a sense that the speaker understood the mistake without delay.

Here что means that and introduces a subordinate clause.

The sentence structure is:

• Я сразу догадался = main clause
• что это не тот адрес = subordinate clause

So literally the structure is:

• I immediately realized that this was not the right address

In English, that can sometimes be omitted, but in Russian что is normally expressed in sentences like this.

Тот here means that one / the one needed / the correct one in context, not just physical distance.

In sentences like this, не тот is a very common expression meaning:

• not the right one
• not the one I meant
• not the one needed

So:

• это не тот адрес = this is not the right address / this is not the address I meant

If you said не этот адрес, it would sound more like:

• not this address as opposed to some other address

That can be possible in some contexts, but не тот адрес is the more natural choice when you mean the wrong address.

Because это sentences often equate one thing with another, and the noun after это is commonly in the nominative.

Here:

• это = this / it
• не тот адрес = not the right address

So the structure is essentially:

• This is not the right address

That is why адрес stays in the nominative singular.

Тот is the masculine nominative singular form of тот, which usually means that. But in this expression, its meaning is more idiomatic.

Не тот + noun often means:

• not the right ...
• not the correct ...
• not the intended ...

Examples:

• не тот человек = not the right person
• не то место = not the right place
• не то время = not the right time

So не тот адрес is best understood as a set phrase meaning the wrong address or not the correct address.

Yes. Russian word order is flexible, although the original sentence is very natural.

The neutral version is:

• Я сразу догадался, что это не тот адрес.

Other possible variants could shift emphasis:

• Я догадался сразу, что это не тот адрес.
• Что это не тот адрес, я сразу догадался.

These are grammatically possible, but they may sound more marked or literary depending on context.

For a learner, the given word order is the best standard model.

It can be either, depending on context.

Догадаться often means:

• to guess correctly
• to figure out
• to realize
• to infer

In this sentence, natural English might be:

• I immediately realized that it wasn’t the right address.
• I immediately figured out that it wasn’t the right address.

Guessed is possible, but in many contexts realized or figured out sounds more natural.

Yes. Both can work depending on the English version.

• это literally often corresponds to this or that
• but in natural English, it is often better

So these are both reasonable:

• I immediately realized that this was not the right address.
• I immediately realized that it wasn’t the right address.

The Russian это is very common in identification statements like this.

Only the past tense verb changes.

• masculine singular: Я сразу догадался
• feminine singular: Я сразу догадалась
• plural: Мы сразу догадались

So:

• Я сразу догадалась, что это не тот адрес. = said by a woman
• Мы сразу догадались, что это не тот адрес. = We immediately realized that it wasn’t the right address

The rest of the sentence stays the same.