Learn Hardcore Romanian: Bunica era fericită când nu aveam febră și ne jucam în grădină. - Grandma was happy when I didn’t have a fever and we played in the garden.

Bunica is grammatically feminine singular (it means grandmother), so any adjective that describes her must agree in gender and number.
Fericit is the masculine singular form of happy.
Fericită is the feminine singular form, so Bunica era fericită literally means Grandma was happy (she-fem.).
If the subject were masculine singular, you would say Bunicul era fericit (Grandpa was happy).

How do we know that aveam and ne jucam mean we and not I?

Romanian usually drops subject pronouns because the verb ending shows the person and number.
Aveam on its own is ambiguous: it can mean either I had or we had, depending on context.
But ne jucam can only be we were playing, because ne is the reflexive pronoun for us / ourselves, not me / myself.
So once you see ne jucam, you know the implied subject is we, and you understand aveam as we did not have a fever.

Why is a se juca (as in ne jucam) reflexive? English just says play, not play ourselves.

In Romanian, the common verb for to play (as children do) is a se juca, which is reflexive.
So you always combine it with a reflexive pronoun:

mă joc – I play
te joci – you play (sg.)
se joacă – he/she/they play
ne jucăm – we play
vă jucați – you play (pl.)

In the imperfect past, ne jucam means we were playing or we used to play.
This is just how the verb is built in Romanian; it does not literally mean we were playing ourselves the way it might sound in English.

Why are all the verbs in the imperfect tense (era, aveam, ne jucam) and not in a simple past like a fost, am avut, ne-am jucat?

The imperfect tense (era, aveam, ne jucam) describes ongoing, repeated, or background actions in the past.
This sentence talks about a general situation or habit in the past: Grandma would be happy whenever we did not have a fever and were playing in the garden.
Using the compound past (a fost fericită, nu am avut febră, ne-am jucat) would usually describe one specific, finished event, not a repeated or background state.
So the imperfect fits better to express a typical or recurring situation in the past.

Does era fericită mean was happy or used to be happy?

It can mean either, depending on context.
The imperfect tense often suggests a continuous or repeated state in the past, so Bunica era fericită can naturally be understood as Grandma was (used to be) happy in those situations.
In many contexts, English can translate it simply as was happy, but the idea of ongoing or repeated happiness is built into the Romanian imperfect.

Why is it nu aveam febră and not nu aveam o febră or nu aveam febra?

With health conditions like febra (fever), Romanian usually uses a bare singular noun after a avea (to have):

am febră – I have a fever
nu aveam febră – we did not have a fever
You normally do not add an article here; o febră sounds strange in this basic medical sense.
Similarly: am gripă (I have the flu), am răceală (I have a cold), generally without articles.

Why is there no subject pronoun like ea (she) or noi (we) in the sentence?

Romanian is a pro-drop language: subject pronouns are usually left out when the verb ending already shows who is doing the action.
Era automatically tells us he/she/it was, and the noun Bunica shows that it is she.
Aveam and ne jucam have first-person plural endings, and ne also shows we, so adding noi (noi nu aveam febră și noi ne jucam) would sound redundant or overly emphatic.

What does când do here, and could we move the când-clause to the beginning?

Când means when and introduces a time clause: when we did not have a fever and were playing in the garden.
The structure is:

main clause: Bunica era fericită
time clause: când nu aveam febră și ne jucam în grădină

You can absolutely put the când-clause first:
Când nu aveam febră și ne jucam în grădină, bunica era fericită.
This is perfectly natural and does not change the meaning, only the emphasis and rhythm.

Why is it în grădină and not la grădină?

În generally means in / inside, indicating being within the space of something: în casă (in the house), în parc (in the park), în grădină (in the garden).
La is more like at / to, often used for locations as points or destinations: la școală (at school), la bunica (at grandma’s).
Since we want to say that we were physically in the garden, în grădină is the natural choice.
La grădină would be unusual here and would suggest at the garden in a looser sense, not actually inside it.

Why is it în grădină (no final -a on grădină) if English says in the garden with the?

Romanian often uses a bare singular noun after certain prepositions where English uses the.
În grădină can refer to in the garden in context, even though grădină is formally indefinite.
If you say în grădina, that is the definite form (in the garden), and it usually points to a specific, identifiable garden (for example, our garden, already known in the conversation).
Both are possible, but the bare form în grădină is very common and natural, especially when the garden is just the ordinary place where you play.

Can nu aveam febră also be written as n-aveam febră?

Yes. In speech and informal writing, nu often contracts with a following verb that starts with a vowel:

nu aveam → n-aveam
nu era → nu era (less often contracted), etc.

So n-aveam febră is a very natural, colloquial way of saying we did not have a fever, and it would not change the meaning of the sentence.