Learn Arabic (Modern Standard): التمر حلو جدا.

In Arabic, التمر is a collective noun. It refers to the general category of dates rather than specific countable ones. Collective nouns are treated grammatically as singular masculine, which is why the adjective حلو is also singular masculine. If you wanted to talk about a single individual date, you would add a taa marbuta to make it تمرة.

Why does التمر have the definite article ال if the English translation does not say 'The dates'?

When making a general statement about an entire category in Arabic, you must use the definite article ال. While English drops the word 'the' to say 'Dates are very sweet', Arabic requires it to show that you are talking about the generic concept of dates. Additionally, the subject of an equational (is/are) sentence in Arabic is almost always definite.

Why does the word جدا end with an alif?

The word جدا acts as an adverb here, meaning 'very'. In Arabic, adverbs take the accusative case, which is marked by a 'tanween fatha' (an 'an' sound) at the end of the word. Unless a word ends in a taa marbuta or a hamza, Arabic spelling rules require adding a silent alif to the end of the word to support this 'an' ending.