आज मौसम अच्छा है, लेकिन कल मौसम ठंडा होगा।

Yes, कल can mean either yesterday or tomorrow in Hindi. The meaning depends on context.

In this sentence, होगा marks the future, so कल must mean tomorrow:

• कल मौसम ठंडा होगा। = Tomorrow the weather will be cold.

If the sentence were in the past, then कल would usually mean yesterday.

Yes, you can leave it out if the meaning is clear.

The full sentence repeats मौसम for clarity:

• आज मौसम अच्छा है, लेकिन कल मौसम ठंडा होगा।

A more natural shortened version is often:

• आज मौसम अच्छा है, लेकिन कल ठंडा होगा।

Hindi often drops repeated words when they are understood from context, just like English sometimes does.

Because मौसम is a masculine singular noun in Hindi.

Adjectives like अच्छा and ठंडा change to agree with the noun they describe:

• masculine singular: अच्छा, ठंडा
• feminine singular: अच्छी, ठंडी

So:

• मौसम अच्छा है = The weather is good
• हवा ठंडी है = The air is cold

Since मौसम is masculine, अच्छा and ठंडा are the correct forms.

है means is, while होगा means will be.

So in this sentence:

• आज मौसम अच्छा है = Today the weather is good
• कल मौसम ठंडा होगा = Tomorrow the weather will be cold

This is the basic contrast between present and future:

• है = present
• होगा = future

होगा is a future form of होना (to be).

For masculine singular subjects, the future often uses:

• होगा = will be

Other common forms are:

• होगी = will be (feminine singular)
• होंगे = will be (masculine plural / respectful)
• होंगी = will be (feminine plural)

Since मौसम is masculine singular, होगा is the correct form.

Hindi usually follows Subject–Object–Verb order, and the verb often comes at the end.

A rough structure here is:

• आज = today
• मौसम = weather
• अच्छा = good
• है = is

So literally it is something like:

• Today weather good is

That is normal Hindi word order.

Yes. लेकिन is very common and natural, but you may also hear:

• पर
• मगर

All can mean but, though लेकिन is often taught first because it is very standard and clear.

Examples:

• आज मौसम अच्छा है, लेकिन कल ठंडा होगा।
• आज मौसम अच्छा है, पर कल ठंडा होगा।
• आज मौसम अच्छा है, मगर कल ठंडा होगा।

They are just spelling variants of the same word.

• ठंडा
• ठण्डा

Both mean cold, and both are understood. In modern everyday writing, ठंडा is very common.

Hindi does not use articles like a, an, and the the way English does.

So:

• मौसम अच्छा है can mean The weather is good or simply Weather is good, depending on context.

Hindi relies more on context than on articles.

Yes, you can.

• आज मौसम अच्छा है = Today the weather is good
• आज का मौसम अच्छा है = Today’s weather is good

The version with का sounds a little more explicit, but the shorter version is very common and natural.

Similarly:

• कल का मौसम ठंडा होगा = Tomorrow’s weather will be cold

ठ is a retroflex aspirated consonant.

A simple way to think about it:

• curl your tongue slightly back
• make a t-like sound
• release it with a strong puff of air

It is different from:

• त = dental t
• ट = retroflex t
• ठ = retroflex th (with aspiration)

So ठंडा is not pronounced like English th in think. It is a Hindi t sound with breath.

Hindi word order is somewhat flexible, but not completely free.

The most neutral version is:

• आज मौसम अच्छा है, लेकिन कल मौसम ठंडा होगा।

You might also hear:

• मौसम आज अच्छा है, लेकिन कल ठंडा होगा।

That can work, but it may place slightly different emphasis.

For learners, it is best to stick with the original order until you become comfortable with Hindi sentence rhythm and emphasis.