Las moras de esta mañana estaban muy dulces.

Las is the plural definite article, meaning the.

• la mora = the blackberry / the mulberry
• las moras = the blackberries / the mulberries

Spanish usually uses articles more often than English does, so las moras sounds completely natural here.

Moras is the plural of mora.

In Spain, mora commonly means blackberry. In some contexts it can also mean mulberry, so the exact fruit depends on context. If the meaning has already been given to the learner, the main thing to remember is that it is a feminine noun:

• la mora
• las moras

Literally, de esta mañana means of this morning.

In natural English, that often becomes:

• from this morning
• this morning’s

So Las moras de esta mañana is basically the blackberries from this morning or this morning’s blackberries.

This is a very common Spanish way to connect a noun with a time expression:

• la reunión de esta tarde = this afternoon’s meeting
• el partido de anoche = last night’s match

Because mañana is a feminine noun when it means morning.

So the demonstrative has to agree with it:

• esta mañana = this morning
• esa mañana = that morning

Even though mañana ends in -a, that alone does not always guarantee gender in Spanish, but in this case it really is feminine: la mañana.

Both estar and ser can translate as to be, but they are used differently.

Here, estaban muy dulces describes the state/condition of the blackberries at that moment: they were very sweet. That is why estar is the natural choice.

• estar often describes conditions, results, or how something is at a given time
• ser is more for identity, classification, or more inherent description

So:

• Las moras estaban muy dulces = the blackberries tasted/were very sweet
• Las moras eran dulces could sound more like a general characteristic, depending on context

For food, Spanish very often uses estar with adjectives like bueno, rico, dulce, salado, etc. when talking about how it is on that occasion.

Estaban is the imperfect, while estuvieron is the preterite.

In this sentence, the imperfect is used because it describes a background quality or ongoing state in the past: the blackberries were very sweet.

• estaban muy dulces = they were sweet / they tasted sweet
• estuvieron muy dulces is much less natural here, because the preterite tends to present the state more as a completed event

With descriptions in the past, Spanish very often prefers the imperfect:

• La casa era grande = the house was big
• El café estaba frío = the coffee was cold

So estaban fits the descriptive feel of the sentence.

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

Here, moras is:

• feminine
• plural

The adjective dulce has the same form for masculine and feminine singular, but in the plural it becomes dulces:

• la mora dulce
• las moras dulces

So:

• estaban muy dulces = they were very sweet

Muy means very, and in Spanish it goes before adjectives and adverbs.

So:

• muy dulce = very sweet
• muy dulces = very sweet (plural adjective)

This is the normal order in Spanish:

• muy bueno
• muy grande
• muy rápido

You would not say dulces muy here.

Yes, you could, but it means something slightly different.

• Las moras de esta mañana estaban muy dulces = the blackberries from this morning were very sweet
• Las moras estaban muy dulces esta mañana = the blackberries were very sweet this morning

The first version attaches this morning directly to the blackberries.
The second version attaches this morning more to the whole situation.

In many contexts the practical meaning is very similar, but grammatically the focus is a little different.

Yes, very common.

Spanish often uses de + time expression where English might use a possessive-style structure:

• el café de esta mañana = this morning’s coffee
• la llamada de ayer = yesterday’s call
• las noticias de anoche = last night’s news

So de esta mañana is completely natural and idiomatic.

Yes. A literal translation helps you understand the structure, but natural English may be different.

Depending on context, natural English could be:

• The blackberries from this morning were very sweet.
• This morning’s blackberries were very sweet.
• The blackberries were very sweet this morning.

The Spanish structure does not always map word-for-word onto English, so it is normal to understand de esta mañana as a time label attached to the noun.

Because of number agreement.

Las moras is third person plural, so the verb must also be plural:

• la mora estaba
• las moras estaban

So estaban clearly matches las moras.

The sentence itself is standard Spanish and would be understood widely, not only in Spain.

That said, a learner of Spanish from Spain might notice that fruit vocabulary can vary by region. In Spain, mora is a normal word, and the overall structure of the sentence is very typical of everyday Peninsular Spanish:

• article + noun: las moras
• time phrase with de: de esta mañana
• past description with imperfect: estaban
• adjective of taste/state with estar: muy dulces

So even if the sentence is not uniquely Spanish, it fits very naturally with Spanish as used in Spain.