Seit dem neuen Wasserfilter kaufe ich kaum noch Wasserflaschen.

Seit usually means since or for when talking about time from a starting point up to now. In this sentence, it introduces the idea since getting the new water filter.

When seit is used in this time sense, it takes the dative case, which is why you get:

• dem neuen Wasserfilter

not den or der.

So:

• seit + dative

Examples:

• Seit dem Unfall fährt er vorsichtiger.
• Seit der Prüfung ist sie entspannter.
• Seit dem neuen Wasserfilter kaufe ich kaum noch Wasserflaschen.

Because Wasserfilter is masculine: der Wasserfilter.

After seit, you need the dative, so:

• nominative: der neue Wasserfilter
• dative: dem neuen Wasserfilter

Both the article and the adjective change:

• der → dem
• neue → neuen

So dem neuen Wasserfilter means since the new water filter.

German main clauses usually follow the verb-second rule (V2). That means the finite verb must come in the second slot.

In this sentence, the first slot is the time phrase:

• Seit dem neuen Wasserfilter = slot 1

So the verb must come next:

• kaufe = slot 2

Then comes the subject:

• ich

So the structure is:

• Seit dem neuen Wasserfilter | kaufe | ich | kaum noch Wasserflaschen

If you start with the subject instead, you get:

• Ich kaufe seit dem neuen Wasserfilter kaum noch Wasserflaschen.

That is also correct.

Because the sentence begins with a different element: Seit dem neuen Wasserfilter. In German, when something other than the subject comes first in a main clause, the verb still has to stay in second position.

So:

• first position: Seit dem neuen Wasserfilter
• second position: kaufe
• then the subject: ich

This is a very common pattern in German and often sounds more natural than always starting with ich.

Kaum noch means hardly any more, barely any more, or almost no longer.

It combines:

• kaum = hardly / barely
• noch = still / any more, depending on context

Together, kaum noch suggests a change compared with the past:

• before, I bought more water bottles
• now, I buy very few

Examples:

• Ich esse kaum noch Fleisch.
• Er schläft kaum noch.
• Wir sehen uns kaum noch.

So kaum noch Wasserflaschen kaufen means buying far fewer water bottles than before.

Yes, noch often means still, but in combinations like kaum noch, it helps express no longer much or hardly any more.

Compare:

• Ich kaufe noch Wasserflaschen. = I’m still buying water bottles.
• Ich kaufe kaum noch Wasserflaschen. = I hardly buy any water bottles any more.

So here, noch is part of the idea of a change over time.

Because German often leaves out the article when talking about things in a general, non-specific way.

So:

• Ich kaufe Wasserflaschen. = I buy water bottles / I buy bottled water (in general)
• Ich kaufe die Wasserflaschen. = I buy the water bottles (specific ones)
• Ich kaufe einige Wasserflaschen. = I buy some water bottles

In your sentence, the speaker is talking generally about buying bottled water, not specific bottles.

This is a very common German pattern. With seit, German usually uses the present tense when something began in the past and is still true now.

So:

• Seit dem neuen Wasserfilter kaufe ich kaum noch Wasserflaschen.

This means:

• the change started when the new filter appeared
• it continues into the present

English often uses the present perfect or a different structure:

• Since getting the new water filter, I’ve hardly bought any water bottles or
• Since I got the new water filter, I hardly buy bottled water

But German normally stays with the present here.

Yes. That is a very natural alternative.

• Seit dem neuen Wasserfilter ... is compact and a bit elliptical, like since the new water filter
• Seit ich den neuen Wasserfilter habe ... is fuller: since I’ve had the new water filter

Both are correct. The version with seit + noun phrase is shorter and common in everyday German.

You could also say:

• Seit ich einen neuen Wasserfilter habe, kaufe ich kaum noch Wasserflaschen.

Wasserflaschen literally means water bottles, and in context it normally means bottles of water that you buy.

So this sentence is understood as:

• buying bottled water less often

If you wanted to be extra explicit, you could say:

• Flaschenwasser = bottled water

For example:

• Seit dem neuen Wasserfilter kaufe ich kaum noch Flaschenwasser.

That may sound even closer to the intended meaning in some contexts, but Wasserflaschen is perfectly understandable.

Yes.

German seit covers both ideas:

• since

  • starting point
  • seit Montag
  • seit dem neuen Wasserfilter

• for

  • duration up to now
  • seit drei Jahren
  • seit langer Zeit

Examples:

• Ich wohne seit 2020 hier. = I have lived here since 2020.
• Ich wohne seit drei Jahren hier. = I have lived here for three years.

So English splits this into since and for, but German often uses seit for both.

It usually goes before what it modifies, here the object phrase Wasserflaschen.

So:

• Ich kaufe kaum noch Wasserflaschen.

This is the natural order.

You may also hear slightly different word orders depending on emphasis, but kaum noch before the noun phrase is the most straightforward and common choice here.

Yes, several.

Some natural alternatives are:

• Ich kaufe seit dem neuen Wasserfilter kaum noch Wasserflaschen.
• Seit ich den neuen Wasserfilter habe, kaufe ich kaum noch Wasserflaschen.
• Seit dem neuen Wasserfilter kaufe ich fast keine Wasserflaschen mehr.
• Seit dem neuen Wasserfilter kaufe ich kaum noch Flaschenwasser.

These are all similar, but with slightly different style or emphasis:

• kaum noch = hardly any more
• fast keine ... mehr = almost no ... any more
• Flaschenwasser focuses more directly on bottled water than on the bottles themselves