Když babička uvidí děti se svíčkami, bude se určitě usmívat.

Here když means when.

• Když often introduces a time clause: when
• In some contexts, especially in spoken Czech, it can also mean something closer to if, but that is not the natural reading here.

So the structure is:

• Když babička uvidí děti se svíčkami = When grandma sees the children with candles
• bude se určitě usmívat = she will definitely be smiling / she will definitely smile

This is because uvidí is from the perfective verb uvidět.

In Czech, perfective verbs do not normally have a true present meaning. Their present forms usually refer to the future.

So:

• vidí = he/she sees, is seeing from imperfective vidět
• uvidí = he/she will see / catches sight of from perfective uvidět

That means:

• Když babička uvidí... = When grandma sees...

This is very normal Czech. After a word like když, Czech often uses this kind of form for a future event.

They are related, but they differ in aspect.

• vidět = to see; imperfective
  
  • ongoing, general, repeated, or uncompleted action
• uvidět = to see / catch sight of / notice; perfective
  
  • one completed event

So:

• Babička vidí děti. = Grandma sees the children.
• Babička uvidí děti. = Grandma will see / will catch sight of the children.

In your sentence, the idea is a single moment: grandma sees them, and then she smiles. That is why uvidí fits well.

Děti is the plural form of dítě (child).

Here it is the direct object of uvidí, so it is in the accusative plural.

The tricky part is that for this noun, the nominative plural and accusative plural are the same:

• nominative plural: děti
• accusative plural: děti

So in this sentence, děti means the children as the thing being seen.

You know it is the object because:

• babička is the subject
• uvidí is the verb
• děti is what she sees

The phrase se svíčkami uses the preposition s / se meaning with, and that preposition requires the instrumental case.

So:

• svíčka = candle
• instrumental plural = svíčkami
• se svíčkami = with candles

This is a very common pattern in Czech:

• s kamarádem = with a friend
• s dětmi = with children
• se svíčkami = with candles

Both s and se are forms of the same preposition.

Czech often uses se instead of s when it is easier to pronounce. Since s svíčkami would be awkward because of the consonant cluster, Czech prefers:

• se svíčkami

This is mainly a pronunciation/sound issue, not a change in meaning.

Compare:

• s bratrem
• se sestrou
• se svíčkami

Here it most naturally describes the children:

• děti se svíčkami = children with candles

So the idea is that grandma sees children who have candles.

Technically, Czech word order can sometimes allow ambiguity, but in this sentence the natural interpretation is definitely that the children have the candles, not the grandmother.

This is another aspect choice.

• usmívat se = imperfective = to smile, to be smiling
• usmát se = perfective = to smile once / to break into a smile

Future of imperfective verbs is formed with být + infinitive:

• bude se usmívat = she will be smiling / she will smile

If the sentence said:

• usměje se

that would sound more like she will smile once or she will break into a smile.

But bude se usmívat suggests a more continuous image: once she sees the children, she will be smiling.

Not really. In usmívat se, the se is part of the verb itself.

So you should learn the verb as:

• usmívat se = to smile

In English, we do not say smile oneself, and in Czech the se here usually is not translated separately. It is just a normal part of many Czech verbs.

Other examples:

• smát se = to laugh
• dívat se = to look / watch
• bát se = to be afraid

So in this sentence, se is not an object; it belongs to the verb.

Because se is a clitic, a short unstressed word that usually goes near the beginning of its clause.

In bude se určitě usmívat:

• bude is the finite verb
• se comes right after it

That placement is very normal Czech word order.

You would not normally say:

• bude určitě usmívat se

because that sounds unnatural.

It can correspond to either, depending on context.

Literally, because it is an imperfective future, it often feels like:

• will be smiling

But in natural English, will smile may also be a perfectly good translation.

The Czech form emphasizes an ongoing or repeated action more than a single completed moment.

So:

• usměje se = more like will smile / will break into a smile
• bude se usmívat = more like will be smiling

Určitě means definitely, certainly, or for sure.

It adds confidence or certainty:

• bude se usmívat = she will be smiling
• bude se určitě usmívat = she will definitely be smiling

It is an adverb, and Czech allows some flexibility in where you place it, but the position here is very natural.

Yes. Czech word order is quite flexible, although different orders can slightly change emphasis.

For example, you could also say:

• Babička se bude určitě usmívat, když uvidí děti se svíčkami.

That means the same thing overall.

The original sentence starts with the when-clause, which sets up the situation first:

• When grandma sees the children with candles, ...

That is a very natural way to say it.

Because Když babička uvidí děti se svíčkami is a subordinate clause introduced by když.

In Czech, subordinate clauses are normally separated by commas.

So the structure is:

• Když ... , ...

This is standard Czech punctuation.

Babička is usually closer in feeling to grandma or granny than to the more formal grandmother.

It is warm, ordinary, and very common in everyday speech.

Grammatically here:

• babička is nominative singular
• it is the subject of the sentence

So babička uvidí = grandma will see